Yes there are footwell vents. You have either a bad vacuum pod at the vent actuator, a bad switchover valve or a defective heater control. Most likely a bad vacuum pod. It's to the right of the accelerator. It is visible once the under dash panel is removed. It can be replaced by working on it from both footwells. It has a bayonet type fitting. A plastic latch must be pushed in and the pod turned to remove it. If you do a search on my posts, I posted instructions how to replace it about a year ago.
Test the pod by putting a Mity Vac on it and see if it actuates with vacuum. There are rebuild kits and new pods available from George ?????.

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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