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Check motor operation by pulling connector and applying 9V with an external power source (battery or bench supply) to the plug.
If the motor turns with reversed polarity then proceed by swapping black and red contacts leading to the motor. With a little flathead screwdriver press the contact spring, pull out the cable.
If the motor only turns by applying external power and a little twist or not at all, only solution is to replace the motor. Mabuchi RF-500TB-12560 is readily available on the internet.