It's the weatherstripping that seals the liftgate when it's shut. This is why it only happens when we're driving and it's raining REALLY hard. Lots of water has to be running on the roof from front to back for it to leak. And even then, it doesn’t leak very much. The weatherstripping leak is in the middle, but the water runs into the channel formed by the edge of the headliner and down into the D pillar like this:



Then the water pools in these areas and runs over down the seatbelt and down into the fuzzy foam on the inside of the bottom of the D pillar panel.



It's interesting that the headliner, where it's molded around the edges, has this hardened material on the inside that seems like a thin layer of dried glue (you can see it in the above pic...it's the part that looks more yellow on the edge of the inside of the headliner). Whatever that stuff is, it kept my headliner dry and made it harder to figure out where my leak was coming from.

If you find water coming down the 3rd row seatbelts or wet spots in the back corners of the cargo area, this might be the culprit. Run a hose over the roof. Pull the headliner down enough to check the roof rack screws. We had the hose going for probably 3-5 minutes before it started leaking the way it does when we're driving through heavy rain.