Reading the post you refer to it sounds like the rear glass exterior rubber weather seal was removed.
Best to find the actual leak location first, however, since leak point and point of water dripping and becoming visible are not necessarily the same place. This would require removal of rear interior window and parcel shelf trim plus the 3rd brake light, to expose the glass/urethane contact surface.
You may wish to also consider getting an estimate from auto glass shop to re & re existing back window glass.
p.s. I believe the arm hole referred to is actually the sub-woofer hole, an option that was not made available in N.A.
