You've established the motor works, now just check the earth from the motor to the body, or wherever it is earthed to is ok, maybe even try a seperate earth run to motor.
Check now for a LIVE at the switch that becomes live when switched igntion to position 2(normal driving), note the colour, switch the switch to spped 1 and see if there is is output on another wire, this will go to the resistor, check this for each speed, note the fastest speed setting is full 12v and may not go to the resistor as it bypasses it dirtectly to fan, if the wires are hot/live when switched to the resistor, then the problem must be between the resistor and the motor, losing all speeds, esp fastest is very unusual, it would be much more likely to be a connection problem at the motor itself, but basic fault finding will find it.
tabetha