As Aus78Formula said if you loose the fan on the low speeds it is usually the resistor. The power for the fan runs through the resistor for speeds 1, 2, and 3. On high, the resistor is bypassed and it gets full power through the relay (located on the side of the A/C housing next to the resistor).
There is a power coming from the resistor that is attached to the relay, but usually if the relay is out you will have the 3 lower speeds and not high. There is also a power wire coming from the alternator going to the relay but it is also for the high speed fan (disruption in power from this wire will only affect the high speed fan - as that was the problem with mine).
I have attached the wiring schematic for you to examine. Hope this helps.