I was dealing with a similar issue in my MY98 R129. Long story short it turned out to be that the light in the trunk was staying on and draining the battery. Turn the cell phone video function on and place it in the trunk and close the lid to see if light is staying on.

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82 380SL 41K miles sold, 98 SL500 44K miles, 13 C300 4Matic, 14 CLA-250 4Matic