cut and pasted from another thread:
ok so this is going to be very simple,
A: make sure that the option is turned on in the menu
B: make sure the polarity setting is right (negative for negative trigger reverse wire, and positive for positive trigger reverse wire)
C: verify that the wire is getting the correct signal with a DMM as close to the avic as possible
if the settings are correct, and the wire is getting the correct trigger and it is still not working, something in the unit is broke.
if the reverse wire was shorted out, and it was a positive wire, there could be a blown fuse or damage to one of the cars systems, and the signal isnt being sent.