Theory Of Operation
WARNING: This page is about a different car, the 2016 Dodge Durango. However, it is still accessible from the selected car via links, so may be relevant.
When the Body Control Module (BCM) receives a CAN Bus message from the Transmission Control Module (TCM), indicating the vehicle is in REVERSE, the BCM provides a voltage output through the Backup Lamp Driver circuit to the Backup Lamps. A ground path for the Backup Lamps is present through the harness to the body sheet metal.