The easiest and most accurate, and probably the cheapest, way to check an AC is have a AC tech add a fluorencent dye in the AC system. If you see it in the leak, your evaporator is kaput!get ready for big bucks$$$
If is a regular water leak, NAPA sells a fluorencent dye that you can add to the regular supply. Drive it for a while and then check with a black light for leaks. I don't know what the MB techs have to say about it.
Good luck!

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J.H.
'86 300E