If you're still looking for a MB shop in Tucson I can recommend Diesel Master on Tuttle. The owner is Marty who has been taking care of my 560SEL since I bought it. Don't let the name mislead you, he handles ALL Mercedes, not just diesels. He ONLY handles Mercedes. I can recommend him. If you want a phone number, let me know (I don't have it on me at the moment I'm writing this.).

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2002 ML320 (thereby making my 560SEL my "spare).
1992 500E, pearl black, 27,000 original miles as of 7/2003.
1990 560 SEL, black pearl metallic, sweetness to drive. Fully outfitted with driving countermeasures (Beltronics radar/laser detector and Uniden BCT-12 Bear Tracker scanner).
1985 Porsche 911 meteor metalic gray, a rocket to drive.
1988 Chevy Silverado pick-up (gotta have something to drive to the rodeo!)
1978 Corvette Indy pace car for show
1971 Opel GT for sentimental reasons