The resistor circuit actually controls the speed of the motor by routing the 12VDC through different “legs” of the circuit, containing different resistor values, depending on where you set the fan control. It is possible that the final cicuit out to the fan is bad. However, if the speeds were working before this failure, I;d suspect the blower fan motor itself.
The blower fan motor will be installed into (sticking through the wall of) your ductwork, down by your passenger’s feet. You can unplug and remove the motor, and test it with a regular 9 volt battery on your kitchen table. It has very low draw, and it should run smoothly and beautifully. I’ve done this more than once.
Be advised that the fan motor screws may be Torx, These are often used by Toyota for this application, apparently it makes the assembly process easier. Look carefully or you’ll be frustrated.