2008 Cadillac CTS water leakage

How common is water leakage into interior cars?
Our dealer will not address water damage. States every make of vehicle has this problem.
We found standing water in our trunk wheel well, as well as water under all carpet, as we removed it.

Your car is ten years old. Over that time, there may be rubber seals that dry out or degrade in another way and allow water to enter the car. Since it is the trunk, the problem might be the rubber gasket around the trunk, the rear lights, or the seal around the rear window. If there is rust in the trunk that opened a hole, water can get in that way too. The car is out of warranty and the dealer can’t help you as a warranty repair.

I would take it to a well known body shop. They should be able to figure this out pretty quickly.