Don't know if this will help, but I had the same problem with a 1986 300E. Same chassis, I figure the same relay box. Replacing the diode was the fix. On the 300E, the diode is a little guy at the very back of a box of relays aft of the fuse box on the driver's side of the engine compartment. It's right up against the firewall next to the hood hinge, as I recall. The part is inexpensive and easy to replace (if you have small fingers). I dropped my failed one between the relay box and the firewall and never saw it again.

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David

His:
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
Hers:
2003 Chevrolet Suburban
1999 E430