Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac CTS. 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 serial data message from the HVAC module indicating that the rear defrost system is active. The door switch supplies B+ through the mirror heating element control circuit when a request is seen. Ground is applied through the mirror heating low reference circuit.