Principles of Operation
WARNING: This page is about a different car, the 2007 Lincoln MKX and 2007 Ford Edge. However, it is still accessible from the selected car via links, so may be relevant.
The trailer lamps are supplied power by the trailer tow park lamp relay, the LH trailer tow stop/turn relay and the RH trailer tow stop/turn relay. They are energized to correspond with the exterior lighting functions of the vehicle. A common ground is provided to the trailer tow connector for the trailer lamps.