Circuit Description
WARNING: This page is about a different car, the 2007 Hummer H2. 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 battery voltage to the mirror heating element supply voltage circuit upon receipt of a class 2 message from the body control module (BCM) indicating that the rear defrost system is active. The DDM and PDM also supply a constant ground for the mirror heating elements.