2001 Dodge Ram AC heater control

I have a door at the heater coil that will not hold. Its all heat or all cold. If i hit the brakes i can hear the door close. If i make a adjustment on the heater control knob i can hear the door close shut. if i turn the fan on high i can hear the door close and i get all heat when this happens. if i turn the fan on low i get the ac back. I have no control over that door. PLEASE HELP.