i had this problem a while back in my "quento" and it ended up being a leak in my rear light, it would collect in the boot and then once i broke hard enough, it would tickle down the back and to the front... also, the door seal was an issue too, wasn't the seal itself, but the door not locking up tight to it, there is a alan-key type screw you need to adjust, move it closer to the inside of the car, and it will shut tighter against the seal...
but seriosuly have a look at the rear lights taking in water
Hope it helps....