probably the drain tubes. I know on my friends 2001 his sunroof ended up leaking into the car because the drain tubes backed up. I assume you can get to them by pulling of the interior trim but because of the curtain airbags, I would take it to BMW.

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"promote the general welfare, not provide the general welfare"

We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.