Do you need a relay?
Most head units have an output to turn on amplifiers, etc. These outputs are designed to turn on a small number of devices, so they provide very little current.
On high-end systems, when many devices (amplifiers, crossovers, equalizers, processors, fans, etc) have to be connected to this turn-on wire the current output might not be enough. If the circuit is overloaded, it can blow a fuse or even damage the head unit.
There is an easy way around this problem: Add a relay.
How many devices are too many? Depends on how much current each device draws and how much current the head unit provides. Check the specifications section of the manuals to see. Typically, pieces of equipment such as amplifiers, crossovers and equalizers draw very little current, since their turn-on switches are either solid state or small relays. If you are hooking other devices that draw more current such as neon lights, fans, actuators, motors, etc, then you definitely need to add a relay. An easy way to tell how much current devices are drawing is to check with a current meter.
The diagram to the left shows the connections required to get the turn-on output. The relay can be located either behind the radio, trunk, or elsewhere in the car. Usually, it is easier behind the radio because wires going to the relay are shorter.
Terminal 87 goes to constant power (+12v). It can be obtained from the same wire where the radio’s memory backup is connected.
Terminal 86 goes to ground (negative wire going to the head unit or to a metal part that is connected to the chassis of the vehicle).
Terminal 85 is connected to the remote turn-on wire output at the head unit.
Lastly, terminal 30 is run to all the components that need to be turned on.
For more information on how relays work, check out the “relays” page in the reference section of this website.