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I have had experience with this. The blower motor runs through the resistor for the lower speeds. High speed bypasses the resistor via a relay. Often the problem is the motor pulls too much current and melts the plug on the back of the switch. It should pull about 6 amps. The last defective motor I changed was pulling 11.2 amps, which has melted the plug a couple of times until I figured it out. The motor sounded fine, so I assumed it was OK. The resistor had gone previously, which i had changed.
If the fan runs fine on only high, I would check the resistor.
If it intermittently works on the lower speeds, I would check the plug. If it is melted, check the current draw on the fan motor.
I was offered a chinese motor for 40% less than a Delco, but opted for the Delco one. It takes less than 30 minutes to change the fan motor.
dmh