Vent Mode
WARNING: This page is about a different car, the 2003 Buick Regal and 2003 Buick Century. 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.