Water ingress is notoriously hard to track down. My wife's last car, a Subaru Justy developed a severe leak into the passenger footwell which I just couldn't find. I had all the carpets out (to get them dry) and still couldn't find the leak. Eventually, with the aid of my son and a hose, I spotted a trickle of water running out from under the passenger door trim onto the inner sill. If I hadn't had the carpets and sill trims out I'd never have spotted it.
The reason for the leak was that the plastic sheet which seals the inside of the door panel had come away for about 1/2 an inch at the bottom so that when it rained, instead of being deflected into the door and running out the drains at the bottom, water was leaking out the bottom of the plastic seal and running down behind the door trim, from there down behind the sill trim and into the footwell.
The leak in your car may be for an entirely different reason, especially since it is a Fiat and not a Subaru, but you never know.