Maybe it's time to take it up the chain. This may involve you "hanging out" at the dealership to speak with the correct person who can handle your (their) problem. Remember to document everything and take it with you as well. It sucks when you can't trust where you take your car. I'm really paranoid with our M as well. Good Luck with your resolution.

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'03 Carbon Black M5
'04 Silver X5 4.4
'04 MB SL500