Why does my heat and air only work on defrost? on 2000 Chrysler Voyager

And when I turn the knob to any other position it still stay's on defrost.

Asked by for the 2000 Chrysler Voyager
Defrost is the default position on the air damper, which directs air to defrost, floor, vent, mix, etc. It's controlled by either electric motors or, more commonly, vacuum lines. It's likely:

A) Vacuum line to the damper has broken or come off.. easy fix, just replace the hose
B) The motor or vacuum solenoid has gone out and will have to be replaced. I can't say exactly where these are located on your vehicle, but many times requires pulling the dash which is around $600 in labor.