I'd also check the tailgate seal is properly seated all round the top and sides - it sits over a body flange and has a sticky black sealant underneath. If you want to pull the trim back to see what is happening you'll need to take the seal off……
Also just above where your water is coming from there is a rubber grommet sealing a hole about 1" in diameter - outside the tailgate seal but behind the tailgate area. Possible that this has somehow been dislodged a bit.
As Peter mentioned it is easy to check the roof rail mount screws as well. The rail is held on by T25 Torx screws (there aren't any screws under the trim covers at either end, only the ones you can see from the top) - take these out and slide the rail forward and it will lift off. The roof is penetrated by the rivnuts and although they are blind maybe there is an issue allowing water in.
You've also posted in an older thread on this issue - Water leak in rear cargo area
(not sure this is good form, and confusing)
and as posted there check the wiring grommets above the tailgate area and outside the seal.