A soaked cargo area or wet tailgate comes from bare metal meeting bare metal — Jeep didn't add weatherstripping where the rear door and glass close against the body. Add it, and the cargo area stays dry.
Why the cargo area floods
Open your tailgate and look: the door is bare metal, and the body it closes against often has no matching seal. Same with the rear glass. Without weatherstripping on both surfaces, heavy rain and car washes push water straight into the cargo space.
Tailgate vs. rear glass
Trace where the water sits. A wet load floor points to the tailgate surround; water high on the glass or headliner points to the rear-glass seal. Both fix the same way.
The fix
Add the correct rear weatherstripping to the tailgate and glass surround so the rear closes against a real seal — picking up exactly where Jeep left off.