Hope you have fixed it. I've read the various threads on the leak issue on the early Jazz.
Just in case it's of any value, Aygos (and variants) had a bit of a habit of getting water in the boot like yours. I've found they are often due to where the rear light cluster connector access hole goes through the body panel. There is a sponge seal between the cluster and body but this is often distorted/flattened and the water runs down from the tailgate gutter, behind the plastic trim panel, and then drops straight through the hole for the light connection. I've fixed them with good quality silicone (Dow Corning is my preference) on the foam seal against the body panel.
Just a thought, if the Jazz is similar.