I used to own a 1995 C220 and now I own a 1986 300SDL with the same exact problem. In both cars when AC is on(& second button from left is pushed), only air blows from corner vents, left-side of steering and right-side of front-passenger. I can imagine it has to do with some vaccum/electronic problem but there must be an easy shortcut to open center vents too. It doesn't have to be the perfect fix as long as I can get the center vents open. Ofcourse the right repair, if easy, is preferable.
I've read some articles about pulling the dash out and changing the 3 actuators on top-left of radio, also other article that said you can reach them by just pulling radio/Climate-control-ashtray out. And third post saying pull the glovebox out to reach the levers on the left of box that open center-vents, then permanently keep them down.
Any ideas to keep the center vents permanently open?