If blower motor works on any speed, then it is fine. All the resistros do is limit current to the motor by dropping the voltage across it. They are placed in series with the motor by the switch. The factory resistor pack is obvious if it is burned out, a good one will have wires coils coated in ceramic insulation:


A blown one will have burnt off stubs of wire sticking out where the coil used to be.

http://www.4crawler.com/4x4/CheapTricks/Blower.shtml#BlowerResistor

If the switch is OK and the resistors are OK, then the only thing left, sped-related, is the wires between the switch and the resistor. The switch masically shorts out parts of the resistor pack to increase the speed. Low speed is all the resistor segments, medium has the first part bypassed and high speed has them all bypassed. So if low and med. are missing, something is keeping those resistor segments either shorted out ot the wiring has been changed to just be to the high side of the resistor all the time.