Most likely culprit is your heater resistor. The way your heater fan works is thusly:
power is applied to the switch, switch selects high, med, or low speed. High is a direct connection to the motor, med is a low resistance between the switch and the motor and low is a high resistance between the switch and the motor. On early cars the resistor is on the front of the heater box, don't know where it is located on a '71 but follow the switch wires and it will lead right to the resistor block. Replace the resistor and you should have all three speeds back. Note after removing the resistor you will see the elements burned out. This could have occurred from a motor near failure. I would take the fan motor out and have it checked by a motor shop to make sure its not running excess current.
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