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Most common reason for only high speed on blower motor is failed Resistor block on the motor housing, or corroded connectors on the blocks plug in.
Resistor block costs $10-$25 depending on year
Video here: www.youtube.com/watch?v=BQ1j98NVgjk
Other place it can fail, but less common, on the back of the fan switch in the cab panel, remove panel and have a look for melted wires on the switch.