I think 'leak' might be a misnomer in this situation--water is collecting and sloshing around in the area. There isn't any water inside the car.
From preliminary research online, this problem is widespread. It also doesn't look like the problem is limited to convertibles. I noticed a thread on this site for a newer model:
2014 V6 rear water sloshing
The accumulation is significant (it sounds like someone is rolling a giant water cooler bottle around back there).
I can only imagine the rust and damage it is causing.