AC/Heater won't switch vents

[felx29]

I have a 2000 Grand Prix, and while the heater and AC works, the different vent options won't switch. It just blows out from the top and bottom, and even when turning the knob to just defrost or just the top vents, it stays the same. The high to low controls work, but it no longer lets me go to the "Max" setting, which is nice on really cold or really warm days?

Is this a fuse? I replaced the AC control box with knobs and still nothing changed. Any input would be appreciated!!

[BNaylor]

Check for a loose or missing accessory vacuum which originates at the intake manifold. It loops by the firewall and around to the passenger side inner fender. You'll see a junction/coupler plastic connection next to the battery and engine compartment fuse box. Check it first. Look for any cracking along the complete plastic vacuum line. From there it inputs the main wiring harness and into the cabin to the HVAC system.

The HVAC vacuum actuators use engine vacuum for control via the climate control head unit.