This might be the same issue I had on my truck, 2021 XLT. The dealer told me its the rear door panels leaking. It leaks in under the door panel into the carpet.
At the same time, ask them to check your sunroof drains, unrelated to the rear but could cause leaking in the front drivers side...
I'm glad I got the 2.7 Ecoboost Payload package. Even with all the options of sunroof, 20" wheels, sound system, etc, I still have 1600 lbs payload capacity. I was kind of worried. It's weird that when buying a truck or trailer no one ever mentions payload and it's importance when towing.