It's more than likely the speed control resistor - Follow the power
leads from the fan motor back, and four or five wires will go to a
connector that is fastened to the side of the heater duct with a
screw.

 Turn off the power before opening this up, and do not test the motor
under power with the resistor pulled out.  The resistor has to live in
the fan airflow with the power on, or it will quickly burn up.  And if
you grab it while it's on, your fingers are going to have grill marks
like a steak.

 The resistor gives you the Low and Medium speeds, and the fan switch
sends full voltage straight to the motor on High.  If the resistor
goes open on the highest medium setting segment you lose everything
below High, if the last segment goes open you only lose Low.

        --<< Bruce >>--