I used to work at the dealership and it sucked when someone came in with this concern. First take it to a dealership WITH a bodyshop that is in house even if you have to drive out of your way, then the mechanic can work with the bodyshop tech if need be. Also you could check and see if you might be missing some body plug in the trunk. I've seen a van missing one that kicked the water up and soaked the carpet.
If you dont have warrantty coverage or dont trust the repair shops, you could have a friend (or yourself) climb in the trunk with a flashlight and go through a car wash a couple of times. A water hose just dosent have the pressure normally to show leaks. I hope this helps you somehow and good luck.