The center vents only come on when the climate control calls for cooling. They default to close. When the CCU tries to cool the cabin, they should open by applying vaccuum to the actuator that you can't get your hands on.
When I had that issue, I used a long set of forecepts to remove the vac line from the actuator. I plugged the line into a vac guage that rested in the glovebox compartment. That allowed me to verify that the CCU was functioning, (it was) then I removed the vac pod and replaced the diaphram. I did manage to remove the actuator without removing the dash, but it has been so long ago I forgot how I did it. I remember it being a PITA that involved pushing the rubber boot that connects the vents to the ductwork out of the way so that I could get another hand in there.
I am pulling the dash on my current project and replacing all the actuators at once, cleaning the evaporator, replacing the radio, replacing the cruise amp, and and other various parts that are hard to get to all at once.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660