This problem is caused by a defective blower speed control resistor, the part is located behind the glove box under the dash on the passengers side of the passenger compartment, it is a small part that is plugged into the air ducting of the Heater/AC for cooling of the resistor, it is about 1 inch high by 2 inches long and is held into the heater duct/case by 3 screws, it is plastic with a short wiring harness comming from it, the wires should be Black and Red.