My first DIY Write-up. This forum has saved me hundreds, possibly thousands of dollars with DIY's like water pump, t-stat, and Cam Pos Sensor. Hope I can help someone back.
This should be pretty easy to diagnose. If you notice a small stream of water on the sidewall of carpet this is likely the issue. If you it's too faint to really tell. Pulling the door panel isn't too difficult and it will be quite apparent once removed.
Remove the long slender trim piece with some no-mar style panel tools and remove the torx screws in the locations above. Once all screws are removed just pull out and lift up the door panel. You will most likely break a clip or retainer during this process. You can buy bags full on ebay for next to nothing. If you don't feel comfortable removing the door handle cable you can rest the door panel on a plastic tote or something next to the door and keep it attached.
Once removed you'll see this large foam piece. I've heard it referred to as the vapor barrier. The bottom half is the most important. If it has any rips or missing butyl rope (that nasty black tar stuff) in the bottom half this is mostly likely the source for your leak. As you can see in the picture above someone has tore mine to pieces. The passenger side looked the same.
Remove the damaged foam piece and beginning the arduous task of removing the old butyl rope. From what I could find online the only thing that removed butyl rope is butyl rope. This stuff sticks to your hands for days so be prepared. Pull and large chunk of old butyl rope and beginning dabbing the rest and it will begin to stick to itself. This is the most time consuming part. Both front doors start to finish on the repair took just two hours.
It should be very clean before applying the new butyl rope.
When applying try not to press too firmly until the foam piece is in place. As you place the foam on use a good amount of pressure to ensure a nice tight seal. You'll feel the rope spreading as you press down. It's a very reassuring feeling.
It should be solid looking like this when completed. Overall I spent about $90 in materials. The foam was $35 a side from BMW Atlanta. The butyl rope was $20 from amazon for 25'.
Re-install the door panel and bring the rain.