Hey guys, I've been having the same issue - had a day off last week so I've stripped the interior out completely.
I've sealed up the pollen filter and drivers side, removed all door cards and sealed the inner units - hey presto, appeared to have done the trick.....alas it hadn't!
So having left the interior and door cards out for the week it's become apparent that the residual water the runs down the seals is sitting along the bottom seal and not draining out the bottom of the door. Plan tomorrow is to adjust the passenger door and see if loosening it slightly will allow this water to drain out rather then building up and then spilling over into the front footwell and running into the back.
Will let you know if that sorts it. You wouldn't notice that this was the
problem if the interior an door trims etc were stripped out completely.