Principles of Operation: Notes
WARNING: This page is about a different car, the 2014 Lincoln MKX and 2014 Ford Edge. 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 over the High Speed Controller Area Network (HS-CAN) to the Body Control Module (BCM) 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.