Principles of Operation
WARNING: This page is about a different car, the 2006 Mercury Milan, 2006 Lincoln Zephyr, and 2006 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
For vehicles equipped with an automatic transaxle, the transmission control module (TCM) sends gear status information to the instrument cluster over the communication network. The smart junction box (SJB) receives transmission gear status from the instrument cluster over the communication network. When a message is sent over the communication network stating the transaxle is in REVERSE, the SJB energizes an internal relay which supplies voltage to the reversing lamps.
For vehicles equipped with a manual transaxle, the SJB supplies voltage to the reversing lamp switch when the ignition switch is in the RUN position. When the transaxle is placed in REVERSE, the switch routes the voltage to the reversing lamps.