The problem is on the Heater Resistor which is located next to the blower motor. It is a little black square with a conector attached to it. it is held in by a phillips head screw. Take it out and see if any of the elements in the resistor are broken. The resistor steps down the flow of electricity for different switch setings. The blower runs on high when there is no resistance in the circuit. Chances are the last two resistors are broken. Bone yard- about five bucks.