Heated Outside Rearview Mirror Operation
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 REAR DEFOG relay controls the operation of the heated outside rearview mirrors. When rear window defogging is requested, the REAR DEFOG relay is energized and B+ is applied to the supply voltage circuit of the mirror heater element, located in the mirror face. A constant ground is also applied to the mirror face. Current will pass through the mirror heater element, warming the mirror, so long as the rear window defogger is commanded ON.