If you unable to resolve the issue, consider calling Mercedes Roadside assistance, http://www.mbusa.com/brand/container.jsp?/owners/clientcare/roadsideassistance/index.jsp#
On two occassions (the first story could have been very ugly, but in the end I became a believer in this service as another resource), I've called the 800 number and was back on the road within hours. I'm a DITY mechanic for most things, but on a weekend while traveling, I needed the part from the dealer and the Roadside worked. The phone call is free, they will have the MB mechanic on Duty call you (if there is one local) and you can discuss fees. Usually it's 1.5 time the hourly rate (minimum 1/2 hour) plus the parts. If a mechanic is not local, they may have another shop (like AAA) contracted through another roadide service who will call. No fee until they show up and start work......