Backup as in backing up a truck. You must locate the reverse wire which is on whens the truck is in reverse. You can follow it from the reverse light at the tail light.
Backup as in alarm siren at a different location like inside the car. I have done this before. I mounted the siern in the front under the hood facing down and the other inside the car. It was really loud. You can do the following and use the term remote for the signal output from the alarm and that will be enough to power both sirens
without burning stuff out. The Fan 1 and 2 exchange for siren 1 and 2 and you should be good.
Connecting Additional Devices to the Remote Turn On Wire
Using a 30 amp SPDT
relay, connect terminal #87 to constant 12 volts positive with a fuse rated to the sum of the additional accessories you've added and the components you need to turn on. (If you have two fans rated at 5 amps each and a neon light rated at 10 amps, you would use a 20 amp fuse plus 200 ma for each amplifier and processor.) Connect terminal #85 to ground, terminal #86 to the remote turn on wire from the head unit, and terminal #30 to each accessory with an appropriate fuse. A fuse (not shown) could also be used between the output of the relay (#30) and the remote turn on wire of the amplifiers and/or processors for extra precaution.