I had complete success in fixing a similar problem with my '87 300D, which uses a similar setup for ACC controls. I removed the unit from the dash, took it apart, and resoldered all the electrical connections for the pushbutton switches and the temperature setting roller potentiometer.
My symptoms were A/C and fan operation with the unit's OFF button selected, and A/C operation in ECONOMY. Now it works perfectly.
I'm getting slowly motivated to do the same thing with my '82 300D, because the fan is mostly dead on everything except DEFROST - deteriorating during my time of ownership from working most of the time in the other modes.
I certainly was under the impression that the ACC switches were entirely mechanical in nature. I originally thought they were vacuum control valves of some sort. They are actually just rather plain electrical slide switches like you'd find on a radio or something electronic. What I think happens is that people are too violent with pushing the switches, especially when their operation starts to fail.
My theory is that the mechanical impacts of pushing the buttons and also having them pop back out when you push another button, is that the solder connection eventually cracks and fails.
Taking the ACC control unit out of the dash is not too complicated. You do have to carefully and slowly pry off the big electrical connectors - they are pretty massive plugs. Then you can pry off the bottom cover carefully by finding all its holding tabs. This should reveal all the pushbutton solder pins. On the '87 300D, each pushbutton has two parallel rows of three pins.
Its really a simple rebuild, and I'll bet a lot of ACC units have been replaced at $300+ with nothing wrong but bad solder joints.
When I do the '82 300D, I'll post some pictures if I get the chance.
Oh, the vent directions may be vacuum controlled - an actuator may be bad - but go for the electrical fix first....