Have you tried pressing the rear dome lights button near the sunroof control? I know it sounds silly, but I've left mine on many a time!

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1982 300D-T (sold)
1988 300E (sold)
1994 E320 Wagon (sold)
1995 E320
1998 E300D-T
1999 E3204M Wagon
2001 E320
2005 E320 CDI