Can't speak for the Stilo, but Fiat do use seals that push over a lip,
water can get under them at the top and come out at the bottom.
I've taken them off, put sealant in the groove that goes over the lip,
no more water (or mould). I used the clear silicone sealant that
comes in cartridges, about ?1.50 for the cartridge and similar for
the gun at the local cut price tools emporium. Another useful tip
is to turn back the floor covering and spread newspaper
(or talcum powder) then look to see where the newspaper gets
wet first, or where the water tracks start in the talc. You can
also put the paper or talc down, then use a hosepipe, if it's
coming from above it'll show up, from underneath it won't.