I dont recall this being a common problem. But I noticed when I got the car it has some rusting under the paint on the rear door just above the plastic trim skirts. Both sides are about the same. I would expect this in the rear fender area maybe, but the doors? Something I need to look for that is letting water in?

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car