Here it is. But... I saw a website recently that had brand new ones about $35~$40 but I cant remember where.

http://www.ebay.com/itm/MERCEDES-SLK300-SLK350-SLK55-R171-SPAR3E-WHEEL-AIR-COMPRESSOR-ASSEMBLY-OEM-/181749764570?hash=item2a512151da&vxp=mtr