There could be several reasons, but the most common when the fan works on high, but none of the other speeds is that there has been a failure in the fan speed control RESISTOR PACK. At the high speed setting, the 12 volt vehicle supply is fed directly to the fan motor. On all the other speed settings the voltage is proportionally reduced by the series combining of heavy wire resistors. The resistor package is inserted into a rectangular opening in the blower plenum chamber [encloses the heater core, and the air conditoner evaporator coil]. Depending on the vehicle, it may be on the passenger or the engine compartment side of the firewall/cowl. It is normally secured in place with two to four screws, and the wiring harness is connected to the pack with a plug on connector. j3h.