Note: This applies to 96-99 Honda Civics only.
Honda’s engineers came up with a wonderful way to make people buy their parts: Wire the factory alarm system and dome light through the factory radio. Remove the factory radio and dome light and factory alarm/keyless entry stop working.
Dome Light
The signal from all the door switches is fed to the factory radio. From the radio, the signal goes to the dome light. When the radio is removed, the dome light does not work since the circuit is “cut”.
Fortunately, there is a very simple workaround this problem. If the car does NOT have a factory alarm/ keyless entry, all you have to do is tie two wires together.
There are two green plugs. Get to the one that is plugged into the radio (if you have both green plugs going into the radio, then you DO have a keyless entry system).
Tie together the Green/Black (at the corner of the plug) and the Green/Red (3rd pin, opposite corner, same row).
Before tying the wires together, test both wires. With the radio plugged in, both wires should get a ground when a door is open and nothing when the doors are all closed. Once you have determined that they are in fact, the correct wires, unplug the green plug from the radio. If the dome light still works, you do not have to worry about it. If it doesn’t, temporarily jump with a wire both pins at the plug. If the dome light functions correctly, then you can permanently jump the wires together. The preferred way to do this is to strip some insulation on the wires, solder a jumper wire and insulate with electrical tape.
Factory Alarm/ Keyless Entry
There is a single circuit board inside the radio that controls both the radio and keyless entry features, so you do need to keep the factory radio.
The only workaround is to relocate the radio under the dash. The easiest thing to do is to get a “Y” adapter for the main harness (white plug). Y adapters are available at any car stereo store where wire harnesses are sold. If they don’t have one, you can buy a male and female harness and make your own y adapter. This way you don’t cut any factory wires and avoid problems with the dealer.
The factory radio will fit under the dash, behind the area where the cup holders/power outlet are. On the white plug going to the factory radio, you need to hook up lights, ignition, ground and power. Do not hook up any speakers to it. Don’t forget to also plug up both green wires to the factory radio.