Had a 1974 Camaro where the high speed on the switch itself burned out every few years. Was not the resistor which is usually the cause. Some have a separate wire for high speed only which may have become disconnected or corroded.
It sounds like the speed control but I would still check the motor to be sure it is good. I believe you can bypass the control and apply 12 volts direct to the motor which in theory will put the motor to high speed. If the motor runs and the control is new, it is most likely a power problem. Either a bad ground or bad positive connection. Could be a bad new control but it seems unlikely.
On the GM designed systems (only one's I've worked on) the switch could be bad
OR: there is a "HIGH SPEED FAN" relay There are two relays one works low and medium(s) via the resistor and the other is the one that kicks it into HIGH by feeding full voltage to the an (Bypassing the resistor) .