The passenger side heater is not working?

There is a door that opens and closes when the heat is changed from defrost to full to bottom. The door is actuated by either a cable or a vacuum actuated device. The door is on the heater box under the dash in the middle of the vehicle. Look for cable or vacuum hoses running from the back of the heater controls to the outside of the box. Sometimes the cables slip out of their holder and will not open the door. Put the heat on and in the defroster, position then put the cable back in its clip.