[Jastel]

As said the boot hinge area can leak, especially if it gets blocked with leaves and crap...but this shouldn't do it while driving along.

Other possibilities are roof gutter rails, was your car ever a cop car or had lights on top etc, sometimes they leak up there under the black strip.

Also had one leak in through windscreen, it filled up back first but access point was actually front windscreen, check the black strip around the edge, the glue is under here, sometimes you can see a line where it is not sealing properly. It is normally on the bottom edge somewhere and while driving the water gets forced up and in by the airflow, hosing it wont show anything.