I had the same problem.The fan started working intermittently, then very slowly even at 'high' speed then failed completely. I took out the panel under the dash, and removed a cover that becomes visible when the panel's removed (easy job). You can then see the fan. I checked the electrical supply with a multimeter, and power was OK. Conclusion - fan motor failure. It's also easy to remove the fan - you'll see screws fixing it in place. I took it to my dealer - didn't have time to fix myself. Actually there's also a 'regulator' or 'control unit' too. I needed both replacing. Turned out expensive, but now works OK and in this weather it's just as well!
Hope this helps.