Trailer Parking Lamps
WARNING: This page is about a different car, the 2018 Ford EcoSport. However, it is still accessible from the selected car via links, so may be relevant.
The BCM monitors the headlamp switch position by sending voltage signals on multiple circuits to the headlamp switch. There is one circuit for each headlamp switch position. At any given time, one of the signal circuits is switched to ground to indicate the headlamp switch position.
When the parking lamps or headlamps position is selected, the BCM sends a position light indication message over the MS-CAN to the TRM to activate the parking lamps output to the trailer tow connector.