I just installed the third new AGM battery into the 1991 560SEL. After installation, the car started right up as it should. After four days, the battery was deader than a door nail. I put it on the charger and it is now holding charge. Please check out the attached photo; a friend says two of the red wires go to the alternator although I have not traced them myself. Even so, I see no reason why a brand new AGM battery would die after only four days.

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