Look under the drivers knee bolster, attached to the side of the blender box is a round actuator, looks like the one attached to the supercharger. While looking at it with car on flip the control to different positions and see if the actuator arm moves.
Well, not exactly sure if its in the same place on the gtp but it should be I think. Mine came out of the bracket that holds it to the blend box. So the whole actuator would move but not move the blend door. Clipped it back in and its been fine.
Guess first you should also make sure the hose is connected in engine bay on vacuum tree on top of supercharger. Check for cracks in hose.
Forgive me if any info is wrong, this is what I would do on my impala. YMMV
