Hi I'm also having the seme problem and I've done some investigation into it over quite a few websites and a few mechanics apparently it the door speaker mount seal . There is a mount that bolts to the inside of the door that has a seal that fails . The speaker then bolts to the mount which I also think has a seal to. When I rains heavy water runs down the window into the door panel by design but when the speaker seal fails the speaker is right at the bottom and the seals consetualy getting wet then drying out causing it to crack . My car been back in under warranty 3 weeks ago and I told them then what the fix was but they fitted the full internal door seal instead hence I still have the problem. I hope this I helpful to you. I had someone pouring water onto the door window as I took a video clip at the bottom of the door under the speaker between the door card and the door. If you have a way I could send the clip to you or try do what I did.
