[Golten951]

If you pull the face off where the slide arms are located, there are three cables back there and the metal retaining clips can fall off or brake, which will cause the flapper door for the heat to stay open. My 82 924 had the same problem. but that cleared it up.

First pull the radio out, then use a offset Phillips head or a stubby to remove the 2 screws on the bottom of the face plate. place all the levers at their cold points, and you can pull them out by hand.
Remove the face plate and you should see the cables in there and the 3 metal clips.
If there are all present and holding the cables you will need to check the cable next to the heater box under the dash.
This cable in on the drivers side about halfway up. the best was to get under there is slide the seat all the way back and lay on it, its a little uncomfortable at first, then your legs fall asleep and its great after that! Some of the cars have a plate blank off on the side of the heater box where the cable is at, but most do not, you should be able to see the cable, follow the cable to the push point and see if the metal clip is still there, if not, use a heavy zip tie to hold it in place.

I hope this helps you out.