Air Delivery: A/C
WARNING: This page is about a different car, the 2003 GMC Safari and 2003 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
When the operator selects A/C, The mode actuator has vacuum applied to it through the Yellow vacuum line, making the vent door open. The defrost actuator has vacuum applied to it through the Blue vacuum line, making the defrost door close and the heater door open through mechanical linkage. A/C is forced ON.