Agree with tghawk, the motor to move the vent door is more than likely bad. There are 2 one on the pass side which is not to bad to replace. The drivers side is very hard to access but can be done after removing the gauge panel and the top of the dash. Check if only one or both are bad by setting pass and drivers both to full heat. If both blow ice cold it maybe another problem and not the vent door motors which I would not expect to go out at the same time.