I'm only familiar with the heating system on a 123 so take my ideas with a grain of salt. In a 123, a vacuum actuator permits fresh air to enter. I don't know the exact relationship to the center vents but it seems to me quite possible that if the actuator for the vents is bad, the actuator for the fresh air could be bad also. The way to tell is to identify it, and pull a vacuum on it.

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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