Your absolutely correct, water should not be entering in there.
It also sounds like a warranty repair issue.
You could start by comparing whether you get more water in, when the car just sits in the rain versus when the car is driven in the rain. That'd provide a clue as to whether the water is being forced in or just running in off the roof and tailgate.
That'd certainly assist the garage to locate the entry point / points.
Loving my DS4 DSport - 1.6 THP160, EAT6 speed auto, Blanc Nacre
"Most Beautiful Car of the Year, 2013"