A few years ago, a vacuum pod failed in the cabin causing all the vents to close unless set on cold AC - this was in the winter. No wanting to tear into the dash (without a spare car) I opted to disconnect the corresponding vacuum line - now all vents are "open" except for the floor vent. (Center vents - hooray!)

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1984 300D "Brown (and Blue) Betty" Over 200k but the odometer isn't counting