I keep getting water where I painted in red. There was a crack in the metal near the top below the weather stripping, which I sanded and filled with epoxy and it's not leaking from that spot. I took the weather stripping back off, unbolted the liftback strut where it connects to the body, and cleaned the whole area off and found no cracks or signs of fatigue in the metal. Took the taillight out as well to check it's seal, which was fine. Oddly, I don't see any water stains or signs water is streaming down any of the inside metal panels.
Observer in the hatch area while a helper outside gently hoses various parts of the car's exterior, starting low and working slowly upward. If it is coming from the roof seams the ceiling liner will have to come off to find the spot where it is coming in. But if it is the roof it is probably from the seam near the hatch hinge.
Finally found it! Gasket holding the rear bumper attachment bracket was split in half, allowing water in.
The piece shown in the picture looks like a grommet. Usually made of hard but slightly flexible plastic. They don't normally seal anything, they prevent abrasion on cables and wires that pass through holes in metal with sharp edges. Is there a rubber gasket associated with it? We had a leak into the cabin on our Accord where the hood release cable passed through a sort of rubber boot which fit tightly into the metal hole and around the cable. In an act of engineering hubris a drain line from the sun roof ends right above it and water was wicking through and around the aged and not as flexible as it once was rubber. The solution there was to goop the whole area with silicone sealant. (The "proper" solution was to get a new rubber piece and replace the old one, but that would have meant unhooking the cable inside the cabin so it could be threaded through the new boot. That car is a 1998 though, and this fix is good enough.)
It's a felt washer of some kind. I did some testing and the bumper stayed on just as tight with 2 of the 4 plastic pieces that pierce the sheet metal removed. So I used thicker felt on the two remaining, and then put foil tape on both sides of the holes on the sheet metal, and then used silicone to seal the 2 remaining plastic bits.
