In my 1996 Buick Park Avenue the air conditioner only blows thru the windshield and floor vents. No air comes from the middle vents?

If the AC/heater air control doesn't work, you most likely have a vacuum line broken. All the air flow doors in the heater system are vacuum operated and without it the air will blow out of the windshield defrost vents. The common place for a vacuum line to break is under the hood where the lines pass near a hot area. The vacuum lines are normally a rigid black plastic tubing about 3/16 inch in diameter. They run across the engine compartment fastened to the section just below the wipers and air vent. The tube goes through the dash, usually near the steering column. When the tubing has been hot for long time it becomes brittle and cracks or breaks. Trace this tubing looking for a crack or break. The easiest repair is to cut out the broken section and replace it with rubber hose of the correct diameter. The hose needs to have an inside diameter that will fit snugly on to the vacuum tubing.

Thanks for using FixYa.

Gaker