I'd look on the fusebox lid where you will see a fan symbol, there will likely be 2 one for the engine fans but also one for the heater blower, check fuses, if no joy check fan motor earth, then if needed pull out switch and check for live to the switch and away from switch.
Often the resistors pack up, but this will only stop the resisted speeds ie 1 and 2, no3 being full speed and full 12 volts so doesn't go through the resistor.
tabetha