Rolle, I can only share my own experience; I had a very similar issue with my Mustang GT in the trunk having moisture,took everything out of it,including my spare and surface fitted mat....found out the leak was coming from an improperly installed rubber grommet at the base of the trunk, right below the spare tire.....took it out, cleaned it and put it back using some rubber paste seal usually used on grommets, problem solved...just make sure the trunk is left empty, and open and leave it in the sun for an afternoon till everything is bone dry....It's a minor inconvenience at worst....
I would also take a look under the carpeting on the floor in the rear seat, if there is moisture there, I can almost assure you it would be coming through a grommet ... you can see evidence of water intrusion around the grommet...take it out and properly seal it....I used a product called Bostic it's almost tar based and dries pliable and non sticky...and black.
Any decent acrylic door seal would work.