I just helped a friend with a 91 190E 2.6 doing the same thing: Hot air out the side vents and windshield while with the AC on, cool air out the middle vents.
There are many posts regarding this and similiar problems. The first and easiest (which fixed the 190E mentioned above) was a cracked vacuum connector tubing, driver's side, right next to the fuse box area. Very obvious. Trader the rubber tubing for another piece and instantly cured it.
Split vacuum hose
Also check out:
More vacuum line problems
Keep us posted,