Benz has a special 14mm tool for these plugs, but you can easily make one buy buying a 14mm allen-key wrench and cut a 1" to 1-1/2" piece of the long end.
You then put that piece into a 14mm socket and the piece that sticks out of the socket will fit your plugs.. you can also use a 14mm closed end wrench on that stubie.. That allows for tight quarters.
The actaul Benz wrench looks like a wrench with the 14mm hex sticking thru the handle, just as the one I suggest.
See them on ebay.
http://cgi.ebay.com/ebaymotors/Mercedes-Hazet-Oil-Drain-Plug-Wrench_W0QQcmdZViewItemQQ_trkparmsZ65Q3a12Q7c66Q3a2Q7c39Q3a1Q7c72Q3a1171Q7c240Q3a1318Q7c301Q3a0Q7c29 3Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem3ef4dcccd3QQitemZ270396083411QQptZMotorsQ5fCarQ 5fTruckQ5fPartsQ5fAccessories
And you still have the 14mm allen wrench for other jobs ...I bought a set of allens and cut 1" off all of them , so I have a set of these little stubbies to fit in each size socket..I am surpriized no one has made these as a packaged set, b/c they allow you to use your regulaer socket set as the drve for the Stubies.
14mm and 17mm are the most common size Benz uses......