Could be a couple of things.
The first and simplest place to check is the plastic membrane that sits between the door metal and the interior door cards. Whip off the door card and see if the membrane is damaged or peeling away from the metal part of the door. It can even be the drain holes at the bottom of the door being blocked.
Other than that it could be a sign of a knackered heater matrix - more involving to fix to be honest.
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1976 Audi 80 L - Sleeping
1977 Audi 80 GLS - W.I.P
1991 Golf Mk2 VR6 Supercharged
2003 Audi A6 Allroad - Daily
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