Ensure again all drainage is clear including below windscreen if there is a gap clear it out with a tooth brush.
Another thought the route of most windscreen washers to rear window is in the roof( I've had this problem) the joint may have parted and each squirt will cause water ingress. The obvious is the rear window gets no water from the jets. Just a thought but worth looking at. Failing this and the other suggestion may be worn rubbers around the doors.
bob, January 2013
