I assume that the system is fully charged and the compressor is coming on?
Have you checked the vacuum pot on the driver's side of the transmission hump inside the car? That's what opens the center vents. Also have you tested the pressure switch on top of the dryer/accumulator? If you bypass that switch and your vents open, it's defective.
The only other thing I can think of is the mechanical actuator in the dash.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."