You were right...one of the door bindings was broken. The leftmost vacuum actuator arm near the driver's footwell was unattached from the plastic clip which is on the rod running through the airbox. Unfortunately the plastic clip which secures it is broken off. Ordered a "flap repair kit" from MB. Hopefully it will include the necessary items to fix.

A couple of the plastic clips in the airbox are extremely poorly designed for this auto. Should be steel bindings. They are very small, would add little overall weight to the car and are of too critical an importance for HVAC function. Not to mention the time and effort involved in taking the whole thing apart to do the fix.

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1984 300 SD 384,000 Miles