What might cause fiat 500 water leak in boot?

I'm sorry to hear You've got a water leak happening in the trunk of Your FIAT 500. Sometimes, the metal can warp, and water can get inside that way. Even weirder, water can actually ingress through the tail lights, if there are cracks or gaps there. In fact, for an odd thing You might not consider, it can come in via the rubber seal on the rear windshield wiper. Fixing it here, or at the tail light, can be done with a seal of silicone grease. Typically, if You see water in this area, it's going to be because the grommets surrounding the trunk have begun to deteriorate due to age and exposure, and simply need to be replaced.