Hello! I'm Rixster-
Can you tell me if this car has a sliding Sunroof?
I would have the seal around the window checked and re-sealed if needed. This seal can crack and shrink over time and cause this. There really is no other way back there to cause such an accumulation of water.
You can check this by drying the trunk area out as best you can and running the garden hose over the window for a time and checking it.
That's my suggestion.
I've seen this a couple of times and was able to have the window shop remove and replace the window to repair it.
I hope this helps...
Thanx! -Rixster
ASE Certified Technician
Owner Rix Automotive & Mobile Repair 27+ Years Exp. with all makes and models cars and Lt. Trucks