Brian is right, the 123 & 124 operate the same way. Only cold air comes out the center. In the winter, forget about them, you should get hot air out the side, floor, and defrost. If you want to force the center vents open for A/C in the summer, it can be done - I did it. I stuffed my hand back there with the glovebox out and forced the pod to open the flap, then stuck a screwdriver into one of the center vents to jam the flap partially open. Then you don't touch the screwdriver! It's ain't pretty but it does work (got my wife through this past summer.) I need to pull the dash & fix the stupid pod properly.
Never done a 123, but I've done three different 124's, here's the photos of that - plus photos of the failed rubber bits:
http://www.meimann.com/images/mercedes/W124_dash/
Regards,