Principles of Operation: Notes
WARNING: This page is about a different car, the 2013 Ford F-550 Super Duty, 2013 Ford F-450 Super Duty, 2013 Ford F-350 Super Duty, and 2013 Ford F-250 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
When the transmission is in REVERSE, the PCM (gasoline engine) or the Transmission Control Module (TCM) (diesel engine) 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 the RUN state.