Circuit/System Description
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The HVAC control module controls the rear window defroster through the REAR DEFOG relay located in the underhood fuse block. When the rear defroster is activated the HVAC control module supplies battery voltage through the defroster relay control circuit to energize the relay coil.