There was a recent thread with a leak in the same place, but on the driver's side, IIRC. I'm not sure if they got to the bottom of it.
If you remove the plastic gutter trim and wipe it out, you might be able to see a crack in the sealant, it is usually obvious when it cracks at the tailgate end. Other possible sources are a leak from the radio antenna tracking across the inside of the roof, that's quite a common leak point, and also the windscreen bond in the corner.