Either the pushbutton unit or the fan speed regulator module, which is mounted on a heat sink below the blower motor, could cause the problem that you describe.

You need to measure the control voltage coming from the pushbutton unit. There is a 3 terminal connector behind the brake booster that has black (ground), red (+12 volts) and yellow (control) wires.

The connector is quite difficult to get to. I made an adapter to measure the voltages but you could remove the plastic connector housing, re-install the terminals and then probe them to check voltages.

The voltage on the yellow wire should vary from 1.0 (low speed) to 6.0-10 volts (high speed) depending on your pushbutton unit settings. If your voltage does change but your fan speed does not then your speed regulator module is defective. If the voltage stays low, then it is your pushbutton unit.