Go under the dash and by the heater yu should see a cople of vacuum motors. Try moving the arm on one or the other while the blower is going and se if it changes the vents from defrost to heater or dash. That is the one thatisn't working. It will eithe not hve vacuum going to it or just stuck as they do that sometimes. If it's notgetting vacuum then it may be the control unit that is bad or no vacuum going to the control unit.