Water Leaking on Rear Trunk Cover.
I had a GM car years ago with a similar problem. The dealership had difficulties locating the problem and after a few months of complaints, it was the wheels spinning in the rain and splashed into a defective underbelly seam that caused the water build up. The rest of the car was dry except this area.
Not sure if you have the same problem, but it is difficult to locate.
Sent from BimmerApp mobile app