Been there, done that. My 240D leaked into the rear floor boards for years, The car was near perfect in every other way. It drove me crazy, that's what's wrong with me now.
You're not going to believe what I tell you, but if you want to take advantage of my experience with this problem, here's what you do:
Open the hood. Under each hood hinge, there is a drain. These drains are like dents in the sheet metal as it curls downward to join the vertical sheet metal of the firewall. Clean these drains with bailing wire. For city people, coathanger wire makes an acceptable substitute for this advanced technology tool.
Now, if you will look inboard from this pocket into which the hood hinges fold, you will see a vertical piece of metal. At the bottom of the vertical metal is an opening to allow drainage inboard. On the drivers side it comes out below the master cylinder, and on the passenger side it comes out below the battery tray. Look closely in this area and you will find two more drains. Now that you have carefully fabricated your ultra precision, high tech drain cleaning device, use it on these two holes as well.
Everyone who I tell this too, does not believe me, but they always come back later and tell me that it worked.