The rear window defogger system consist of the following components:

Battery positive voltage is supplied through the RR DEFOG fuse, in the underhood fuseblock, to the DEFOG relay switched input. When you turn ON the ignition, ignition voltage is supplied through the SEO IGN fuse, in the underhood fuseblock, to the DEFOG relay coil. When you depress the rear window defogger switch. The HVAC control module illuminates the rear window defogger indicator and sends a class 2 message to the BCM to enable the rear window defogger system. The BCM energizes the DEFOG relay by grounding the control circuit of the relay. 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 grid. Ground for the rear window defogger grid is provided by G301.

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