Only one speed on the heater blower motor is the switch bad are the motor
The resistor would be bad...by the blower motor, follow the wires back from blower motor.
It is highly likely that the Fan speed resistor module has failed. This resistor pack limits the voltage to the motor in stages resulting in controlling the fan speed depending on the switch position. If all you have is one speed, I would suspect the fan speed (resistor) module.
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