Circuit/System Description
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The driver door module (DDM) and passenger door module (PDM) supply voltage to the outside rearview mirror heater elements upon receipt of a message from the HVAC control module indicating that the rear defrost system is active. The door module supplies B+ through the driver/passenger mirror heating element control circuit when a request is seen.