The rear window defogger system consists of the following components:

When you depress the rear defogger switch, voltage is supplied to the REAR 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 REAR DEFOG relay switched input and the REAR 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.

When you start the engine and press the rear defogger switch for the first time, the defogger cycle lasts for 10 minutes. Further defogger cycles vary based on ambient temperature and vehicle speed. The rear window defogger will be extended when the ambient temperature is cooler or while the vehicle is in motion.