I am not an electric expert, I only have read this
If you want to repair it, you have to replace the 4 (four) MOSFET's TO220, housing BUZ 71 can be used.
http://www.bimmerboard.com/forums/posts/136145
Another comment on a German board says (I translate this as good as I can):
"I would not use BUZ 71, because they only have 45 Watt, the BUZ 100 would be better, it has 250W and 60A. The fan max. pulls 30A, that are at 12V 360W, at 14V 420W. Based on this 2 x BUZ 100 are better than BUZ 71,
4*45= 180 Watt.
However, at low fan speed the MOSFETs become warmer then at highest speed. Or better use BUZ10 / 11."
Now it is the turns for the electric experts.
Have no sword, but from all info I've seen, ...
they are in parallel, so four of them are 56 amps, blower can not be that much.
Also doubt the mosfets are fried, would check soldering points as suggested. The must probable status a blown mosfet will end in a circuit like this one is short-circuited, so blower symptom for burnt mosfets would be running at max speed allowed by the progressive control (not max stage).
Javier