Principles of Operation
WARNING: This page is about a different car, the 2013 Ford Mustang. However, it is still accessible from the selected car via links, so may be relevant.
The PCM monitors the transmission gear status. When the PCM detects the transmission is in REVERSE (R), the PCM sends a message to the Body Control Module B (BCM-B). When the BCM-B receives this message, it provides voltage to the reversing lamps.