Principles of Operation: Notes
WARNING: This page is about a different car, the 2012 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.
The Body Control Module (BCM) monitors the input from the stoplamp switch. When the brake pedal is applied, voltage is routed to the BCM and the PCM. The BCM then supplies voltage to the stoplamps.
Aside from the stoplamp operation, when the brake pedal is applied, voltage passes through the BCM fuse 31 (5A) to the Trailer Brake Control (TBC) module and a separate circuit for customer use.