Are you talking about the footwell heat? Footwell heat is controlled by a vacuum pod just up and to the right of the accelerator pedal. But my recollection is that one pod controls both sides so it seems odd that one side gets heat and the other doesn't.

Oops, I just saw it's an SD. I was talking about a D.

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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