I think you're both on the right track.

Remove the glove box liner and you will see a vertical manifold by looking to your left and down from the glovebox opening.

Remove the vacuum lines from the manifold (switchover valve) and test them with an aux vac source. I don't have the list at hand telling which is which, but one of them should actuate the defrost pod which can be felt to the left of the glovebox and rearward toward the rearward face of the dash, just to the right of the center of the dash.

If you get no movement of the arm on this pod by applying vacuum to all of the lines one at a time, then that pod is bad. Fortunately, it's the only pod that you have a chance of replacing without complete dash removal, a 15 hour job.

Hope this helps