Circuit/System Description
WARNING: This page is about a different car, the 2006 Buick Lucerne. However, it is still accessible from the selected car via links, so may be relevant.
When the PRNDL is placed in the REVERSE position, a GMLAN signal is sent from the transmission control module (TCM) to the body control module (BCM). The BCM then applies voltage through the backup lamps voltage supply circuit to the tail lamp assemblies illuminating the backup lamps.