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I know with the 5 series there is a plate on the left side (I'm talking passenger side on uk right hand drive) of the car under the bonnet and acting as a cover between engine compartment and passenger compartment that is poorly designed and a seal,actually floods the interior. It goes around the corner into the inner wing. about 200mm x 200mm 90 degree piece of plastic. Few bolts and it's out, you cut a slot and refit!
The other common one is the door membrane which can usually be detected by a wet underside to the door card. What ever I would advise the kick panel out and pull the carpet back and get a wet vac in there as the last one I looked under had inches of water covering subs etc
Maybe different in your model but hopefully some starting points
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