HVAC Control Module
WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Rear Defrost Switch: The scan tool displays On/Off. When the rear window defogger switch is depressed, the scan tool displays will momentarily display On.