I can't seem to get heat or air to come out of the two center vents in my '89 300CE. If I turn the temperature dial down towards the cold side to engage the A/C, I do get air coming out, but it stops after I turn the temperature dial back towards warm/hot. I do get air out of the two corner vents on the dash when the defrost or bi level setting is selected. Is this a vacuum problem or should I be looking for something else?