Circuit/System Description
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The Driver Door Switch (DDS) and Passenger Door Switch (PDS) supply voltage to the outside rearview mirror heater elements upon receipt of a serial data message from the HVAC module indicating that the rear defrost system is active. The door switch supplies B+ through the driver/passenger mirror heating element supply voltage circuit when a request is seen. Ground is applied through the driver/passenger mirror heating low reference circuit.