Air vent problem--1988 300E

I have a 1988 300E with 223,000 miles. The car is perfect except for one problem that crept up in the last week: When the AC is engaged, cold air blows only out of the windshield defogger vent and the left-most vent to the dash. Nothing whatsoever comes out of the center dash vents or the righ-most dash vent.

Any ideas as to the source of my problem? Is there a malfunction in the pushbutton control unit that is causing these vents to be closed off from inside?

And, yes, I have manually opened the center and right vents with the wheels next to the vents.

Thanks in advance for your help!

Ed