I had a similar problem, when stripping the rear interior out it became apparent my car had had a rear end crash in the past. The back panel had not been pulled out enough so there was a gap between the boot lid and the back panel which was letting water in

That's an extreme case but I tried resealing the bumper vents to no avail. Does your rear window washer work? It's common for a join in the pipe to come apart at the top of the c-pillar, when you try and wash your rear screen it just runs down into the boot.