Vent Mode
WARNING: This page is about a different car, the 2002 Oldsmobile Intrigue. However, it is still accessible from the selected car via links, so may be relevant.
When the operator selects VENT, the A/C Bi-level mode actuator has vacuum applied to it through the Blue vacuum line, making the vent door open. The heater/defroster actuator has vacuum applied to it through the Red vacuum line, making the defrost door close and the heater door open through mechanical linkage.