I've had similar heating oddities in my 85300td. I'm pretty sure my problems lie in the electronic control mechanism. Try using the different buttons to see if things change. Sometimes if mine cools down on the far right heat button, I can get heat if I push one of the other buttons. Sometimes if I put it on defrost, it will solve the problem.
Have you been glowing more than once in the cold weather and waiting for 20 seconds or so after the glow light goes out?

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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