Most likely its one or more of the rear door vapor barriers coming loose at the bottom.
Don't be afraid pull the door cards...it's a skill we all need ... Glue the broken plastic bits back on with RTV and clamps.


Not likely to be a sunroof drain but I suppose it's possible.

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'99 E39 528i 5-speed 130k
'06 325Cic auto 115k
'05 X5 3.0 auto 173k-SOLD
'03 530i - manual swapped - 430k miles-SOLD