Dogtown - not sure where you're located but if there is another dealer in your area, give them a try. Many times if you tell them upfront you have a Amex cert of part of a Fleet program, they will build that into the price even though they are usually reimbursed by MB and they make the same profit.

Tell the dealer to go fly a kite and walk out. If they want the sale they will come after you. Happened to me when I used my corporate fleet pricing program. Our internal memo tells us to negotiate the price on our own and then present the cert/program code for the additional discount that MB will reimburse the dealer for. The salesperson brought in the "Fleet Manager" who tried to use some BS lines on me about how I'm not entitled to use the fleet discount unless I'm buying 5+ cars or pay the sticket etc. I said fine - I will just go to "___ MB" where they offered the same deal AND willing to accept the fleet pricing certificate.