Two causes, usually -- a bad climate control fuse (low speed blower only, heat only out the defrost no matter what button you push) or a vac leak.
First thing to check is the condition of the vac lines in the engine compartment. The green hard plastic line is the one for the climate control. If it is bad, you get air out the defrost, floor, or side vents only, no center vents.
The system has to hold vac. Best thing to do is head to the auto parts store or Sears and buy and MitiVac hand pump. Nice little gadget, you can then hook it up to the green line and pump away. If the center vents now work, hook up to the other engine end and find out why you don't have vac.
Most common cause is a dead rubber connector. These can be replaced by the MB part, or by simple vac line (1/8") from the autoparts store.
Occasionally a hard plastic line will break -- they get brittle with age -- you can splice it with vac line.
If you have good vac on the engine side, you will need to go hunting under the dash for the vac servos. Some are accesable on the W107, I think, but I'm not sure. You may have to do some dign ito the guts of the dash. If so, I'd replace all of them, or at least the diaphrams, to avoid a repetition of a long job later.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!