I have had problems with the contacts in the left rear tail light assembly too. I think it was corrosion, because when I took all the bulbs out and hit the contacts with some very fine grit sandpaper, it went away. Hasn't returned for about five years now.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles