The rear window defogger system consist of the following components:

Battery positive voltage is supplied through the REAR DEFOG fuse to the REAR DEFOG relay switched input. Ignition voltage is supplied to the REAR DEFOG relay coil. When you turn the ignition to the ON position, and depress the rear window defogger switch. The HVAC control module sends a class 2 message to the rear integration module (RIM) to enable the rear window defogger system. The RIM energizes the REAR 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. The RIM then sends a class 2 message to the HVAC control module requesting the illumination of the rear window defogger indicator. The HVAC control module illuminates the rear window defogger indicator. Ground for the rear window defogger is provided by G304.

When you turn ON the ignition and press the rear window defogger switch for the first time, the defogger cycle lasts 10 minutes. Further operation results in 5 minute defogger cycles. The defogger cycle resets to 10 minutes when you cycle the ignition to the OFF position and then back to the ON position.

The rear defogger feature will not time out if the vehicle speed is above 48 km/h (30 mph).

If the battery voltage is very low, the RIM will execute a load shed management control algorithm for the rear defogger feature. In Load Shed Level 1, the rear defogger is cycled at 80 percent on and 20 percent off of its time. If Load Shed Level 2 is needed, the rear defogger is cycled at 50 percent off and 50 percent on of its time. For more information about Load Shed, refer to Body Control System Description and Operation in Computer/Integrating Systems.