Principles of Operation: Notes
WARNING: This page is about a different car, the 2012 Ford Pickup, 2012 Ford F550 Super Duty, 2012 Ford F450 Super Duty, and 2012 Ford Cab & Chassis. 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.