I have seen - in one car a fault with the boot (trunk) light switch. It was somehow causeing the boot light to stay on. Im not sure how. Also check the glove box light switch. Although I doubt it would be these. Have you checked to see if the alternator is charging correctly?

I have had on a 300E an alternator that was showing it was charging by voltage - but when it heated up the brushes were arcing and causing it not to charge - a result of dust from the brushes.

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1985 190E - Gone
1989 230E - Gone
1995 C200 - Gone
1993 280E 120000KM - Built like a brick, the best one yet