Had a similar problem...it's the radio...even though it's turned off....drains the battery....just enough. About impossible to diagnose. I got the heads up about it from the MB parts guy I use. Get a new radio/sound system.

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'89 260e (212K Mi.), '92 400e (208K Mi.), '92 400e (not a misprint) (146K Mi.), '95 C220, '81 240D--Sold