Anytime an MVAC blower only works on high on any vehicle it is either the switch or the blower resistor. Both can sometimes die of old age but usually it is because the blower motor itself if aging and pulling too many amps. The resistor is also cooled by the airflow from the blower so if any part (cabin filter, heater core, evaporator, blower wheel) is clogged with dust it will overheat and fail from lack of cooling.
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