I have the same problem and I just gutted my car from the driver seat back including the passenger seat. I will soon find out where the problem is comming from but I have work so it will be a couple of days until I get back to you with the results. It would be best to strip out the back including you back seat and you rear pannels, close the lid and have a friend soak the back w/ a hose. then try to find the leak befor you start buying anything. a $50 rubber seal is a lot of $$$ to spend only to find out that it was not the problem. or even worse $150 on new tail lights. it's about an hour and a half job if you have the time and you only nees a 10mm, 12mm and an extertion set, a flat and phill screwdriver and you are ready to start. If you do this and run into any problems give me a E-mail.
P.S.- the back seats come out w/ a clip on each side of the fold up part.
Good Luck and I'll post my results when I find out what happened to my leak.