Could be draining down the glass door sills on the rear door from behind the door card, I would spray water down the glass exterior & see if it leaks through the door itself from behind the door card.
The drainage is done via the water going along the rubber sills at the bottom of the glass when its fully up, the water runs along inside the rubber seal channel & then drops down beind the speaker inside the door & into bottom of door through a hole in bottom of the doors.