I have the same problem with my 1985 300TD. I have noticed, though if the interior gets very warm inside, the center vents work. I haven't figured out if it is the center vent actuator or the one the control in the control panel.

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David McDonald
1981 240D
1985 300D (wrecked)
1985 300TD 365,000 miles
1969 Mustang Fastback