wher is the air pump located and is it hard to change out

Retail cost of the air pump is about $800 from Mercedes, try Dormin, A1Cardone or www.rockauto.com, only takes .5 hour to replace. It's right on top of the engine. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info. There is also a company that independent auto repair shops buy their information from that the same information inexpensively to do it yourselfers. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online The air pump relays went bad and caused the pump to run longer than they should there is a bulletin about that problem.