Once you take the door panels off there is a black "foam" material that BMW used as a vapor barrier. You would think this is just a vapor barrier however it lets a tremendous amount of water IN the truck of the seal around this barrier is broken anywhere. I check each one of mine and a few spots had seperated from the door. I used black sealant from autozone to re-seal this to the door and my water leak issues went away (at least until one of my moonroof drain pipes popped out of the drain tube and leaked behind the dash....)

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2003 X5 3.0i
Premium & Sport package
Steel Gray Metallic
55K miles and counting