Check to see if line #4 on the switchover valve (behind glovebox on W124, it might be behind the control panel on your car) holds vacuum. If not, then... you need to probably replace the center vent vacuum element. It requires removing the dash, unfortunately.

On my W124, this vacuum element has failed. Instead of pulling the dash, I removed the center vent nozzles and held the door open with a piece of plastic. Air always flows through them now. The vent can be closed with the slider wheel on the nozzles if I don't need air coming through. This is a good alternative if you don't want to go through the troubles of removing the dash.