97 LeSabre heater / AC problem

I have a 97 Buick LeSabre that came with dual climate control.Problem is blows AC and Heat only thought the lower heater vents and upper windshield defrost. Will not blow through the AC vents or any other combination. I checked the vacuum coming from the engine and is recieving vacuum into the passenger compartment.

You may have a leak at the control unit or bad vacuum motors. This will require a lot of under-dash work to track this down.

Thanks Busted Knuckles.This would be my 4th time taking the whole dash down to work on heater related problems.I had replaced the control unit about 6 months ago. Vacuum motors are the actuators? Would you know the procedure how to diagnose?