You've probably got a leak associated with one of the vacuum actuators that operates only on heat. The one that immediately jumps to my mind is the footwell ducts actuator. That one only works when the heat is on. Do you have air coming out in the footwells when the heat is on? My guess is that you don't and that is where the problem lies. That particular actuator is directly to the right of the accelerator behind the ducts. To access it, remove the kick panel under the driver's dash. You will be able to access that actuator and the one for the defrost vents.
If that actuator is not the problem, perhaps it is the one that decides whether to bring in fresh air or recirculate the air. That one is behind the glove box somewhere. Remove the glovebox and you will see at least two actuators back there. I think the one I refer to is to the right.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13