Circuit/System Description
WARNING: This page is about a different car, the 2009 Saturn Vue. 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 DEFOG relay. The voltage level should be low while the DEFOG relay is de-energized. The voltage will be near system voltage when the HVAC control module energizes the DEFOG relay.