Some cautions have to be taken when working on alarms. ALWAYS put fuses on wires going straight to the battery (fuse should be as close to the battery as possible). If the wire shorts out somewhere along the line, the fuse will blow. If there is no fuse, and there is a short, wire will get extremely hot, melting insulation and possibly catching your car on fire.

The most common way to see if a wire has power is to use a test light. It is highly recommended particularly on newer cars to use a voltmeter, since test lights will draw too much current and could potentially damage car computers or air bag control modules. Be careful when you are probing around, specially close to the steering wheel. A professional installer knows where all the wires are in a car, and will do an alarm in 1 to 4 hours, depending on the complexity. If you don’t install alarms for a living it will probably turn out to be an all-day project to install an alarm.


It is a good idea to install all the separate components of the alarm, and find all the wires you are going to tap into first and then run wires. To go inside the car from the hood could be really difficult in some cars. Get a light and look at the firewall for unused rubber grommets. If you find one, take it out and drill or cut a hole. Start the wire through the hole, and put the grommet back in the car. Grommets are there for a reason: stopping water from getting inside the car. If it is impossible to put the grommet the back in, seal the hole with silicone, or something similar.

If you could not find an unused grommet, see if there is enough room one one that is already used to cut a hole, and run wires in. Try to get a flat screwdriver throughout it to poke a hole (be VERY careful not to poke into other wires, or tear up anything inside the car (you should have checked inside the car first to see where that grommet was). One most cars there is thick padding (noise damping material) lining the firewall, so you might have to cut a little hole in it too. If you have no luck with existing holes, you might have to drill your own as a last resort (make sure you check both inside and out where the hole is going to be so that you don’t drill into wires, fuel lines, hydraulic fluid lines, or any other part. Don’t forget to put silicone or other sealant once you run the wires.

A technique that is very helpful when running wires is to use thick solid copper wire (get 3 or 4 feet of home electrical wire at a hardware store). First run the solid wire through the hole or across the car under the carpet. Tape up the wires you are going to use for the alarm to that wire, and pull. It is a lot easier that just trying to run skinny wires that bend easily in places you can’t get your hands in.

Always put wire loom over wires (specially under the hood). If not available, tape wires up, and keep them away from moving parts and sharp metal edges. If somebody breaks in your car and tries to disable the alarm, it should be as hard as possible for him to find the wires. When putting the car back together be careful not to drive any screws through wires. It is a very good idea to check that everything in the alarm works properly before putting the car back together.


The multi-tone siren can be customized to play some, one or all the tones, by accessing the IC inside that controls the different tones. Also used are air horns, mechanical sirens, and the car’s factory horn.

The most common location for the siren is in the engine compartment. Sirens should be mounted facing slightly down (so that water will drain), as far away from the exhaust system and the engine as possible. It only takes two people about 7 seconds to break the glass, pop the hood and cut the siren wires, so try to hide the siren best you can with the available space, making sure noise won’t be blocked off. It would be also a good idea to install more that one siren under the hood, and another siren inside the car (there are sirens that are very small and loud, designed specifically for that purpose).

If you really want the alarm to call attention, add airhorns and/or make the alarm beep the car horn. There are two ways of doing this: If your alarm has an alarming output, hook it up to the airhorn or mechanical siren through a relay. If you don’t have such an output, maybe you can use the starter interrupt (usually -) and siren output (usually +) to make them work. To beep the car horn, hook up a relay as before, but using the light flash out of the alarm, so you will get intermittent noise. The car’s horn is usually negative out of the steering wheel and goes to a relay under the hood on most cars, in which is converted to a positive pulse that goes to the horn itself. If you don’t think you are skillful enough to find it at the steering wheel, go to the horn itself, it is just an extra wire to run.

Starter Disable

The most common place to get the starter wire is at the steering wheel. It is usually a fat wire coming from the ignition switch. To test for the wire (volt meter recommended instead of test light), turn the key to the on position (don’t start the car yet). Start testing the wires one by one. If the wire has +12v, then it is not what you are looking for. If you find a wire that has no voltage, crank the car. If the voltmeter shows a voltage temporarily, you found the wire. Cut the wire off and try to start the car. If the car still starts, then reconnect the wire and try another one.

Look at the “relay applications” section for details on the wiring. When done hooking up wires, insulate with electrical tape (you want it to look as stock as possible), hide the relay somewhere, and make sure wires are not close to sharp metal or moving parts when you put the dash back together.


Try to mount the LED in a highly visible location. The best place for an LED is on top of the dash. Another good place is close to the radio (that is the first thing thieves look at). If you want to get more sophisticated, you can install multiple LED’s (usually wired up in series, but no more than 2 or 3, depending on the alarm).

One LED on each door next to the locks is a neat way of letting thieves know there is an alarm in the car. If you are really into calling attention, there are LED scanners commercially available, which are made of a bank of LED’s that flash in different sequences.

Valet Switches

Valet switches should be concealed, but easily accessible. Good locations for them are inside the glove box, next to fuse box housing (be careful drilling there), or inside pockets and consoles. Once you found a location for the valet switch, make sure that there is enough clearance behind the hole you are going to drill. If installing in glove box or pocket, make sure it will be in an upper corner, or location where it won’t get damaged or switched by stuff hitting it.

Pin Switches

Even though most alarms have shock sensors that will go off when glass is broken, it is a very good idea to install a switch under the hood in case the shock sensor does not work (some people use hole punches to poke glass, and in most cases won’t trigger the alarm. Don’t forget to also install a switch in the trunk and doors that don’t have factory switches you can tap into. If you have the voltage sensing type of alarm that does not require the door switches to be wired up, it is smart to hook them up anyways, just in case your light bulb inside the car burns up.

Try to look for the wires starting at the rear doors (if four-door), or at the passenger side door, if two-door. A lot of newer cars have diodes that let current go only one way into sensors that only work with driver’s door such as seatbelts, buzzers, and dash door open displays. See the “diode applications” page for more info on getting around these problems.

Testing wires for factory pin switches: Factory switches at the doors are usually positive for Fords, and negative for the rest of the cars (with some exceptions). If, for example, you have negative door switches (a ground is present every time the door is open), then the switch makes your test light turn on (which would be logically hooked up to +12v on the other end). If you shut the door, the light should go out. Make sure all the doors work on the wire you are testing. If not, you might have to tap into more wires and add diodes.

Installing switches: Make sure that you have enough clearance below, and the hood, trunk or door will in fact hit the switch. Double check back side of a panel when drilling. An easy way to check for clearance PlayDough or foil on the intended switch location. Shut the door, hood or trunk, and see if it got smooshed at least 1/4 inch.
If you are installing a trunk switch, make sure it is not outside of the rubber stripe that goes all around the trunk. You don’t want to drill a hole and put a switch in a place where it will get wet, and water drip in your trunk, making the switch malfunction. If necessary, get a metal brace and build your own switch holder.


An alarm is as good as the sensors that make it go off. If a sensor is not adjusted properly, your neighbors will hate you. There are many kinds of sensors, the most common ones being shock, glass breakage and microwave sensors.

Alarm sensors usually require a positive and ground wire for power, and have one or two outputs for triggering the alarm. If you are adding more than one sensor to an alarm input wire, you might need to diode isolate them to avoid undesired interactions.

Shock Sensors: Typically mounted on, or close to the steering column (be careful with car’s moving parts damaging sensor). You don’t want the sensor too sensitive, after all, you want it to go off when somebody breaks your window, not when a truck goes next to the car. Set the sensor to the least sensitive position (usually counter-clockwise). Either hit bumpers with a rubber mallet, or kick the tires. If alarm does not go off, then make it a bit more sensitive, until desired position is found (lots of trial and error here). Remember that motion does not affect shock sensors, so if you move the car, it will not work, it needs a hit (shock).

Microwave Sensors: Ideal location is in the center of the car, under center console (as long as not obstructed by metal objects). Microwaves will go through fabric, plastic, wood and glass, but not metal. Start by setting at maximum sensitivity and get away from the car (shut all doors and roll up windows). Start walking towards the car. If the alarm goes off before you even get to the car, then make it less sensitive. It will take a lot of tries to get sensor set in the “perfect” position. Once you limited the field to the inside of the car, double check by running your arms across the roof, sides, etc.

Glass Breakage Sensors: Usually consist of two parts, the microphone and the main module. The module can be hidden anywhere under the dash. The microphone shouldn’t be concealed and should be mounted equidistant to all windows ideally. To set, start with least sensitivity, and tap the glass with the side of a quarter (be careful not to break the glass).

Motion Sensors: They have come a long way from the pendulum sensors used years ago, which used to give a lot of false alarms, especially when the car was parked uphill. Make sure you don’t set the sensor sensitive enough to make the alarm go off on windy days. Follow instructions provided for mounting. Typically they are mounted to a solid metal surface (i.e. firewall).

Door Locks

If your car has door locks, by adding a couple relays or using the ones built in some alarms, you can make your locks go up and down with the alarm. Usually it is best to go to the switches in the driver’s door, and run wires inside the car. There are many different types of switching systems: Vacuum type on German cars such as Mercedes (you will need to buy extra accessories), 5 wire alternating (most common), positive, or negative. You will have to check at the switches to see which type you have. Check the “door locks” section for detailed explanations and wiring diagrams.

If you don’t have power locks, actuators can be added to convert your manual locks into power. One actuator per door is required. Two relays are needed to operate the locks. You could also add a two way switch to operate the locks manually from inside the car.

Trunk Popper

If you have a trunk popper activated by a button, then you can just find the wire that gets either a (+) or (-) when the button is pushed, and use a relay, hooked up to the alarm. If you don’t have one, or have a manual trunk popper, then you can add a solenoid at the trunk switch to open it. Then wire it up using a relay.


There are many things that can be added to customize an alarm, such as remote car starters, battery backups, extra sirens, airhorns, sirens with built-in battery backups, window roll up modules, hood latches. You can even purchase kits to convert your manual windows into power windows. Basic installation techniques apply. Follow instructions that come with the products for installation.