It sounds as if you have a problem with the vacuum used to open the air flaps inside the dashboard. Have you ever gotten a blinking light on the front of the heater panel? If you have, it's unfortunately in the auto-climate-control unit and you might have to buy a new or used one.

If not, the vacuum check valve in the hose from the engine compartment may be bad, or you could have a torn diaphragm on one of the actuators that opens and closes the main air flap. This is the same symptom that occurs with Turbo's upon acceleration, when the check valve goes bad: fan runs, air never comes out. I have seen rips in the vacuum actuator just next to the gas pedal, under the center of the dash by the firewall. Not a nice one to get at but at least you can see it without disassembling the whole world.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 805K total