Circuit/System Description
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The heated mirrors are connected to the DDM and PDM. The DDM and the PDM receive a GMLAN serial data message from the HVAC control module indicating that the rear window defogger switch has been activated. If conditions are correct, the DDM and PDM will apply battery voltage to the outside rearview mirror heating element control circuits, and ground to the low reference circuits.