just wanted to share my experience and some pictures, hopefully it will help others.
My rear passenger floor have been soaking wet after going thru the carwash and after heavy rain, so I did some research on how to fix it. Apparently, the 2 common culprits are the vapor barriers in the doors and the sunroof drains. I checked the drains and they were fine. I decided to remove the door panel and take a look at the vapor barrier.
I followed this youtube video:
Part 1: X5/X6 (E70/E71) 2007+ Stage I Speaker Upgrade Install - YouTube
I discovered that the southwest corner of the vapor barrier was coming apart. The butyl tape was still there, but it just wasn't sticking anymore. I pressed it back in place and used Prematex black silicone from walmart to seal it back up. I went ahead and checked all around the vapor barrier and made sure it will sealed tight and not coming off.
I knew I had to remove the carpet to dry out the foam underneath to prevent mold and bad odors. I used some microfiber towels to absorb all that water. There was plenty of water there too.