The rear window defogger system consist of the following components:

Battery positive voltage is supplied through the REAR DEFOG fuse, in the underhood fuse block, to the REAR DEFOG relay switched input. Ground for the coil of the REAR DEFOG relay is provided by the BCM. Battery voltage is supplied from the IGNITION 30A MaxiFuse to the supply voltage on the rear defog relay coil side. When you depress the rear defogger switch, the rear defogger switch pulls the signal circuit low as an input to the BCM. The BCM interprets this as a request for the rear defogger system. The BCM enables the rear defogger system by supplying ground to the REAR DEFOG relay coil. The relay coil ground is also spliced off to the ground circuit of the rear defogger indicator. The REAR DEFOG relay is energized and the rear defogger indicator is illuminated. With the relay energized, battery positive voltage is allowed from the relay switched input through the switch contacts and out the relay switched output to the rear defogger grid. Ground for the rear defogger grid is provided by G401.

When you start the engine and press the rear 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 to the on position.