Why is my floor wet behind the seats after a hard rain with no OBVIOUS leaks?

It is best to look for water leaks when everything is dry. First is the door seal or door, damaged in anyway? I have seen the seat belt having damaged the door seal on many vehicles. Do the doors close flush with body? Ensure the door seal makes good and uniform contact with the door (particularly important if the vehicle has been involved in an accident. You may need to pull back carpets pull back the carpets and sit inside the car (you may need a flashlight) and get a friend with a garden hose to spray water around the door jams to determine if the water is entering the doorway. Then spray water around the windscreen moving around the circumference of the windscreen. Look around the windscreen area and look up from the floor mat area (as best you can) to determine if water is leaking in around the windscreen seal. Is there any rust around the windscreen area or body work? If you have a sun roof there is a drain tube at each corner of the roof to channel away water. A blocked drain may cause water to fall from the sunroof area Water can also come up through the floor pan as you drive through water on the road, if any rubber bung or “seam sealer” has been disturbed or is missing.

Lense cover of the third stop light on back of cab cracked. Had same problem and that was the problem