There could be atleast 5 different reasons why the DSC button is illuminating and we could speculate until we get banned from this forum for over posting. The first thing you should do is to scan the X and see what codes it throws at you. Many of us had issues with the DSC and it was fixed in various ways. Let us know what you find.

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BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers