With the engine off, find the purple vacuum line under the hood. Pull it off the component it's on, could be a vacuum tank or intake manifold vacuum, and attach a hand vacuum pump on it. If your hose is long enough, you can get in the car and manipulate the hand pump as needed to maintain your fake vacuum signal and operate your HVAC controls. Otherwise have an extra person pump it as needed to maintain a good system vacuum.
Move it to the different positions, and listen out for pod movement, and especially AIR LEAK NOISES. You likely won't hear them if you run the engine, this is why you want to do it with a hand vacuum pump or similar.
When you go to any mode except OFF, Heat or Defrost, your upper air duct vacuum actuator should see vacuum. The door shifts and the air should come out the top vents. Put it in OFF, Heat or Defrost, the actuator vents and the door shifts and it should go out the defrost ducts.
If it does not act exactly as described, check the upper door vacuum actuator pod for proper operation (dark blue vacuum line going to it). If you hear air leak noises, track those down and find out what is leaking. If the pod is toast, it's a royal pita to get to.
Plan on this taking some time. Be methodical. It's going to take a bit. Hutch made a great video on how to check the HVAC pods using this method. Of course, his was all torn apart at the time putting it back together, but you'll get the idea. 8:35 mark.
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