Air Delivery Description and Operation (2.2L and 4.3L): Defrost
WARNING: This page is about a different car, the 2003 GMC Sonoma, 2003 GMC Jimmy, 2003 Chevrolet S10 Pickup, and 2003 Chevrolet Blazer. However, it is still accessible from the selected car via links, so may be relevant.
When Defrost is selected, air is delivered to the windshield outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Yellow vacuum line. The mode actuator will retract, closing the Vent Door. The defroster actuator will retract, opening the Defroster Door and close the Heater Door through mechanical linkage.