Circuit/System Description
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2008 GMC Yukon XL, 2008 GMC Yukon, 2008 Chevrolet Tahoe, 2008 Chevrolet Suburban, and 2008 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
The HVAC control module monitors the voltage level on the supply voltage circuit of the REAR DEFOG relay. The voltage level should be low while the REAR DEFOG relay is de-energized. The voltage will be near system voltage when the HVAC control module energizes the REAR DEFOG relay.