DEFROST Mode
WARNING: This page is about a different car, the 2002 GMC Safari and 2002 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
When the operator selects DEFROST, the following occurs: The mode actuator has vacuum applied to it through the Red vacuum line, pushing the vent door closed. The defrost actuator has vacuum applied to it through the Blue vacuum line, pushing the defrost door open and closing the heater door through mechanical linkage.