Vehicle has been owned by the same owner since 1988...no mods. I also asked him about possible incompatible components.

Battery and alternator have been tested (both good). I was thinking it might be something electronic (i.e. the power seat module draining the battery on my former 1997 E420).

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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