I didn't have that exact problem, but I did have a water leak due to (I think) the seal around my hatchback not being 100% tight.
Check all your door seals for pinches or for anything caught in them, even as thin as a piece of paper. For a water spot in that location I'd check all along the top edge of your rear passenger side door first, then the front passenger side door, then the hatchback.
If you don't see anything obvious, take a hose to the shut windows one at a time to see if you can isolate where the leak is coming from. That's basically what the dealer will do.