MBUSA is generally of no help - I have had some minor problems (involving much smaller amounts) with a dealer, and they just don't help!! You have got to get legal with them. Make it clear to them that you will NOT give up until you are satisfied. They should feel that you will go to any extent if need be. 9K is a fair sum of money. The credit card suggestion is great - if its been less than 60 days, you SHOULD dispute the entire charge, simultaneously complain to BBB, and any other agency of relevance (some have been pointed out in this thread). The more noise you make about it, the more of an irritant you are to the dealer, hence he has a greater incentive to work out things. Of course, you must contact the owner of the dealership.
Many of the mercedes dealers are b@#$%&@$, they need to be taught a lesson!!
Good luck, and keep us posted on how it goes!