Principles of Operation: Notes
WARNING: This page is about a different car, the 2013 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.
When the transmission is in reverse, the PCM sends a message to the Body Control Module (BCM) over the High Speed Controller Area Network (HS-CAN), indicating the transmission is in reverse. The BCM provides voltage to the reversing lamps when it receives the message that the transmission is in reverse and the ignition is in RUN.