Just to update... P/N 2028207310 is compatible with P/N 2108206210. Just bought this one on Ebay for $41.98: Free shipping and ONE year warranty to boot!

http://www.ebay.com/itm/RU567-Mercedes-SLK320-SLK350-AMG-A-C-Fan-Blower-Motor-Resistor-NEW-2108206210-/151141015915?pt=Motors_Car_Truck_Parts_Accessories&hash=item2330b4e16b&vxp=mtr
After looking up the MFG. P/N SMP RU567 I found the same part made by Standard Motor Products listed on RockAuto for $179.99.