All vents (including footwell) are controlled by vacuum actuators. It is typical for these to leak. Do a search on 'center vents' and you will find tons of fixes including replacing the actuators. The rubber diaphragms for the actuators are available from Performance Analysis 865-482-9175. About $5 each.

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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