The rear window defogger system consist of the following components:

Battery positive voltage is supplied to the RR DEFOG relay switched input. Ignition voltage is supplied through the IGN E fuse, in the underhood fuse block, to the RR DEFOG relay coil. When you start the engine, and depress the rear window defogger switch. The HVAC control module illuminates the rear window defogger indicator and energizes the RR 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 RR DEFOG fuse and then to the rear window defogger grid. Ground for the rear window defogger grid is provided by G420.

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