I had a similar issue with my '06 Picanto which took a while to track down. Eventually I traced it to a little bit of rust where the self-tapping screws fixed the number plate to the hatchback. The fit had become loose enough to allow rain to get past the screw threads. I cleaned up the area and used a 'penny' washer (sealed with waxoyl) behind the hatch trim to let me firmly screw in a self tapper to secure the number plate. Problem solved.