tiger...same thing happened with my jeep, the fan would only work on high speed, the lower speeds use resistors to lower the voltage and speed, but being resistors they get hot so have a thermal cutout fuse mounted above the resistor block that blows when they get too hot to prevent a fire and stops any power going through the resistors.

I took the resistor block to an electronics supplier, they gave me a new thermal cutout fuse for 97 cents that crimped in and everything was fine again. (the dealership wanted $75 for a new resistor block)

One of those wierd things that if it happens again you know what it is

Pete