The rear window defogger system consist of the following components:

Battery positive voltage is supplied through the REAR DEFOG circuit breaker in the fuse block to the rear window defogger relay switched input. With the ignition in the ON position, ignition voltage is supplied through the HVAC fuse in the fuse block to the rear window defogger relay coil. When you depress the rear window defogger switch, the HVAC control module illuminates the rear window defogger indicator and grounds the control circuit of the rear window defogger relay. This energizes the rear window defogger relay and allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger grid. Ground for the rear window defogger grid is provided by G401.

When you start the engine and press the rear window defogger switch for the first time, the defogger cycle lasts for 15 minutes. Further operation results in 7.5 minute defogger cycles. The defogger cycle resets to 15 minutes when you cycle the ignition to the OFF position and then back to the ON position.