Electric Rear Defogger
WARNING: This page is about a different car, the 2003 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
When the rear defog switch input circuit 292 to the BCM is momentarily switched to ground, as when the rear defog switch is pressed, the BCM will respond by grounding circuit 193, the coil of the rear defog relay.
This action energizes the rear defog relay providing power to circuit 293 - rear defog heater elements, circuit 683 - heated mirror relay coil and circuit 683 - rear defog LED. Momentarily switching circuit 292 to ground again will result in the BCM turning the rear defog relay OFF.