That charging/battery warning light works with voltage comparator. The light gets battery voltage from fuse F1 to instrument cluster. On the other side is the line from alternator voltage regulator. When the voltage coming from the battery is higher that the line coming from the alternator, the warning light illuminates. For test purposes disconnect the small plug from alternator so you see if it's a cluster or voltage regulator problem.

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey