Ok... So I took out the doorpanels on both sides, I saw that it was wet on the inside, and also it looked like the "water protective plastic had been removed and fitted a 2. time. Really bad work, with black tape to hold it on place. After just 3 sec with spraying water from a hose, I saw the water coming down the door and into the car!
So tomorrow I'm going to buy 2 new protective plastic things and fit them.... I also took out the backseat and saw water under this, but after spraying a lot of water on the rear end window, I couldn't see any more water comming in. I also took out the mat and I felt under this, it was as dry as it could be.
So... That means that water from below is not the case and also water won't get trough the mats (talking about the black mats that's fitted from factory of course)
I'll give you all an update on how it goes.
BTW, Daweg360: I was told that on many places under the car it's holes and these are ony filled with, err... not silicone but something else.. And this may get loose over the years. So, be aware that even if your car's not wet on top of the mats the car COULD have a waterleak from below... Since of course the mats are waterproof.