I'm not familiar with the W124 so I can't help you there.
Your best bet is to start a new thread with a subject like 'W124 AC vacuum actuators' or something like that.
If the W124 is anything like the W126, you remove the lower dash panels and the console side panels (you have to look for the screws buried in the carpet). That will expose the inner workings of the AC. I don't know which of the (possibly) 4 actuators you'll see is the center flap actuator. I think it's the lower one in the center. Another thing you should see is a box with about 7 vacuum hoses leading to it. This is the bank of vacuum switches that control the actuators. Some quick testing with a MightyVac will tell you if a swtich or actuator is bad.