Mine had a leak that was repaired under warranty. Unfortunately I was unable to get a great explanation of what weather stripping they replaced since the dealer outsourced to a third party "water doctor".
I think the issue on mine were the corners of the rear window were channeling the water incorrectly causing it all to drain into the rear of the bronco. I found leftover weather stripping similar to what's in the picture below. Just my best guess though.
Either way take it the dealer and let them figure it out.