Next stage of our build. Warning: Lots of Questions...

[BMW135]

Thanks a bunch! That's a really great idea.

The dealership we take it to for certain things said the floor was soaked (we've had flooding rains here lately). They had to pull the seats out to get everything dry. *sigh*

[328 Power 04]

You can easily do that at home.

Also the most likely cause of the leak is at a plug at the top of the A pillar seal. It's a little triangle plug thing, that needs to be resealed with some black silicone. Very common.

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Since water got in, i would suspect the yaw sensor or the lateral acceleration sensor. Both of which you can do at home.
Yaw sensor you can get used from e46 (or else pay ~$1000 for new), and lateral acceleration sensor is cheaper.