I have a 2014 RRS and early last fall I had a water leak in the rear of the vehicle with water dripping out of the D pillar into the cargo area. Not the same symptoms as your leak BUT the cause of my leak could be the same as your problem.
After looking very closely and taking some of the interior trim apart, I found the leak. Water was bypassing the rubber grommets where the wire harness for the boot enters the vehicle. You can see the grommets when you open the boot... they look like black rubber corrugated hoses. Those rubber tubes have grommets at both ends: one entering the ceiling of the vehicle and one entering the top of the boot lid. In my case the rubber grommets were loose in the sheet metal holes of the roof. Its possible that you have the same problem but at the end of the tubes where they enter the boot lid.
I suggest you (or your body shop) look at all the "holes" in the boot lid starting with that wire harness.
Some silicone caulk completely fixed my problem, so the repair was very quick, permanent and easy.