Hmmm.......Fresh/Recirc Flap???
I'm not sure if yours still has this, but in the glove box there is an oval plastic plug. Try removing it and with a good flashlight you should be able to see the fresh/recirculating flap move with different ACC selections.

I believe replacement of that vacuum pod runs about $50. (Note: I believe Brian Carlton mentioned that the replacement diaphragms from autoluxmbz won't work for the long-throw pods, ie...defrost & recirc.)

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )