Circuit/System Description
WARNING: This page is about a different car, the 2017 Cadillac ATS. However, it is still accessible from the selected car via links, so may be relevant.
When the headlamp switch is placed in either the park or low beam position, ground is applied to the tail lamp signal circuit to the body control module (BCM). The brake pedal position sensor provides an analog voltage signal to the BCM. The BCM sends this message to the trailer lighting control module. The trailer lighting control module responds by applying battery voltage to the left tail lamp and to the right tail lamp control circuits. This energizes the tail lamps.