Principles of Operation: Notes
WARNING: This page is about a different car, the 2013 Ford Fiesta. However, it is still accessible from the selected car via links, so may be relevant.
The Body Control Module (BCM) supplies voltage (and ground for the courtesy lamps) to the interior lighting system. The interior lighting system illuminates the courtesy lamps to enhance visibility of the interior when a vehicle door is ajar. The BCM controls all interior lighting functions and timing by monitoring inputs from the door ajar switches, luggage compartment lid/liftgate ajar switch, door lock switch, input from the Remote Keyless Entry (RKE) system and the ignition state.
The voltage supply circuit to the BCM for the interior lamps is also used for the horn.