Yeah using the part from the W140 will work fine as long as you lengthen the leads. I had the same condition, even on high there was only low-medium fan speed...so I waited and on eBay I found a genuine MB part someone was selling still sealed in the bag...picked it up for $130 I think. It comes with the heat conductive grease and a larger heat sink than the old one had so it will probably last as long as the car will now.

As for the location of the cabin filters in the W210, they are accessed from behind the glove compartment, you do have to remove it. There are 2 of them...plus a charcoal filter but it is expensive and it's not necessary to replace it unless you use it. The cabin filters will be grey to black if they haven't been replaced in a year or so...the new ones are snow white.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz