Automatic Transaxle
WARNING: This page is about a different car, the 2011 Mercury Milan, 2011 Lincoln MKZ, and 2011 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
The PCM (2.5L or 3.0L engines) or the Transmission Control Module (TCM) (3.5L engine) monitors the transaxle gear status. When the PCM detects the transaxle is in REVERSE (R), the PCM or TCM provides ground for the reversing lamps relay coil.