What did they say they did in way of "rust proofing"? MB uses a wax based rustproofing. If the inside of the fender is coated in undercoating, there isn't much more you can do.

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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