I had the SAME problem. New battery that would die after a few days of not being driven. The problem, after 2 weeks at the shop was that there was power being leaked through the stereo system. The becker radio was on even though I didnt know it was, and that was draining up the power.

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87' 300E champagne - Euro headlights and corners, 8-hole rims, 140,000miles/220,00km.