I had the same prob after installing my amp/sub. took the battery out and load tested fine. I installed a powercap and havent had a prob since. I don't know If I actually solved the prob but from what I understand the mercedes vehicles are particular about power management.

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Owen

87 300TD 603 I-6
94 Jeep XJ I-6 4.0 (see the pattern)
89 750il
seventy-two hundred feet above sea level (kinda cold up here)