91 AC Interior Vent Problem

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AC cool air stops coming out of top dash vents when an unseen door flapper unpredictably changes position. You can hear a clumping noise when the flapper changes position. I turn the AC off, wait 15 seconds, turn it back on and air comes out again. However, it soon stops again. Also, accompanied sometimes with odd shuddering noises from dash area. The AC interior vent fan keeps blowing and the rear AC unit continues to work. How difficult is it to access the interior vents/flappers? 1991 Grand Caravan still running pretty well. Thanks.