Bosch Example 2: Dual Power Source Switch Wiring

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Pin 30: Wire this directly to the device that you are wanting to power.
Pins 85 and 86: One of these needs to be connected to the ground switch and the other needs to be connected to the power source, pins can be reversed. (see note 1)
Pin 87: This pin will be connected to your new positive source.
Pin 87a: This pin should be connected to your factory circuit that previously proviced power to the device you are adding a back-up power for. (see note 3)

1) Pin 85/86: If you wire pin 85 (or pin 86) to a power source that is always on (i.e. your battery) then if you leave your switch on the relay will provide a slight drain on your battery and will be powered on. Wire this to a power source that is on when your car is on, unless you want to have the ability to turn the powered device on and off even though your car is off.
2) Pin 87: This should be connected to an ignition switched on/off circuit (a connection that is off when the car is off), unless you want your backup device to power on when your car is off.
3) Pin 87a: This pin is the pin that is not available on the 4-pin style relays. Internal to the relay, pin 87a connects directly to pin 30 even if no other wires are connected to the relay, it will stay connected until the relay is switched on.