I’ve fixed a couple of these, the usual culprit is the mastic type sealant that’s in the boot shut at the end of the roof rail the 1/4 panel joins the roof, around the hinge and under plastic strip on the roof and they are absolutely notorious for leaking, just pull up the roof rail, clean out as much of the bad sealant as you can and then tiger seal it up and tiger seal the roof rail back down, job done. Obviously check around the rear lights, as well but I pretty much guarantee it’s the sealant being cracked beside the roof rail.