Whereabouts behind the seat? Down the firewall or under the floor carpet? Mine is soaking under the floor carpet, but it is coming in through the windows and finding it's way around.
Ironically, I need a dry day to sort it out.
I used to get this problem. I found the problem was here;
I solved it by sliding the rubber on the roof slide back down slighly - it had risen up a little [I'm really trying to aviod the double entendres here] and then also padding out the rubber on the car side by putting some foam inside the stip to push it out a little.