The european W124 has got a valve in the vent opening that can be manually closed and opened. If the vent-opening knob cannot rotate freely, the valve that is attached to the knob is not set properly.
If you take a look through the vent grating, you can see an "imbus"bolt that is attached to the valve that can be closed. This bolt has to be untightened to get the knob rotating again (the bolt is probably tightened entirely or the valve is mounted 180 degrees wrong); the valve can be "fine tuned" by this bolt...
The bulb in the knob has to be replaced. There is a screw at the side of the dash lid and there are 4 clamps (2 up and 2 down) visible in the vent opening.
Take out the screw, release the "imbus"bolt inside the vent gratings and stick some hooks under the clamps after which the vent opening can be pulled forwards. The upper clamps have to be bended down and the bottom clamps have to be bended up.
with a minor 600+k