Rear Window Defogger 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.

When you depress the rear defogger switch, voltage is supplied to the DEFOG relay coil and the HVAC control module also illuminates the rear window defogger indicator. Battery positive voltage is supplied at all times to the DEFOG relay switched input and the DEFOG relay coil is always grounded. This allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger. The HVAC module also sends a message to the driver door module (DDM) and passenger door module (PDM) on the GMLAN serial communication circuit to activate the heated mirrors, if equipped.

When you turn ON the ignition and press the rear window defogger switch for the first time, the defogger cycle lasts 10 minutes. Further operation results in 5 minute defogger cycles.