I've also read about the resistor ? Is this for lowspeed fan only.

Also, can anyone tell me if the W210 has only one relay (for both speed), or separete relay ?.

When I unplug the sensor from the thermostat, should I jump them ?.

To take you guys advice, I connect the fan to the 12v source, and the highspeed fan come out (yep, the loud & powerful one), so I'm very hesitate to hand my car to my mechanic to have the whole fan set replaced on Monday. I have the lowspeed fan when the AC on, and highspeed when I jump wire directly to it, so I can't see how the "highspeed" motor fail in the fan set as he said. The repair will cost $500.

Anywhere else I should look at ? should I bring it to another mechanic. My mechanic has been very nice & reasonable to me though (in term of service & price).