Checking the vacuum actuator for the footwell vents is pretty easy. Remove the trim panel under the driver's dash. The actuator for the footwell vents is just to the right of the accelerator behind the ductwork. Hook a Mityvac directly to the actuator and pull a vacuum. If it holds a vacuum, its ok. Then put the mityvac on the line going to the actuator, warm up the car, turn on the heat and see if vacuum is getting down there to the actuator. It there's no vacuum getting to the actuator it is either the switchover valve or the CC unit itself or a leak in the lines connecting those things.
My footwell vents weren't working and it was the rubber connecting hose right at the actuator itself. Fix took a couple of minutes once I traced it down.
While it could be an electrical problem, with all the vacuum lines in that system I'd put initial focus on a vacuum problem.
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