Hi Saintetienne.
I have just got my car back from Fiat. This is what they have done it may help you.
If you open the rear hatch back. Look at the top R/H corner of the frame, outside of the hatch door seal to the right of the door hinge right in the top corner you will see the end of the roof rubber moulding. Just below this on the rear frame you will see where the roof, side and rear panels join together. There is a small tab of metal facing you about 10mmx7mm, which is the end of the roof panel.
At this point look for the rear seam where the roof and rear panels join in the top corner. This seam of sealant fails. If you remove the old sealant and re seal this seam and all seams in the top corner this should fix the leak.
I have some photos of the repair if this will help.
Good luck Andy