If the blower resistor goes bad, the fan will run at a very low speed. If your car is an earlier model W210, the resistor is no longer available. You'll either have to

1) Adapt a W140 regulator (which looks somewhat similar to the small square w210 regulator.
2) Buy the larger new regulator, wiring harness adapter kit and a blower motor from a 2000+ W210.

I did the later, it cost me about $250 in parts.

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With best regards

Al