I'll try and get a pic up tonight mate...
Just another thought - I had a similar problem with my very first car. I had passengers complaining that the back seat was wet but there was no way water could be getting in. However when I popped the boot and lifted the carpet, it was clear to see. The numpty who owned the car before me had drilled a hole in the boot floor to (presumably) let any water out that got in (must've had a similar problem in the past), but it made things ten times worse. The spray from the road was coming up through the hole, filling the spare wheel well and trickling forward everytime I used the brake before coming to rest underneath the rear seats.
Have you lifted the carpets in the back and looked for any holes or anything like where water could get in???