Circuit/System Description
WARNING: This page is about a different car, the 2010 GMC Sierra, 2010 GMC Cab & Chassis Sierra, 2010 Chevrolet Silverado, and 2010 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
When the driver presses the brake pedal, the switch contacts open and B+ is interrupted to the body control module (BCM). The BCM then supplies voltage to both the turn/stop lamp supply voltage circuit and trailer rear turn/stop lamp supply voltage circuits. The BCM also supplies a control voltage to the center high mount stop lamp (CHMSL) relay energizing the relay and illuminating the CHMSL.