You know, I never thought about the issue regarding the insurance company. The car is indeed considered a 2 seater, and having more than what the car was designed for does raise some legal issues. I guess maybe finding a SLC would be a better, since it has a proper backseat, albeit some one. As for that dealership, I am only aware of 2 Benz dealerships, Cherryhill and Princeton. Which one were you thinking of?.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member