Hello, I have been investigating why the rear passenger footwell carpet behind the driver has been getting wet. In order to get the trim out it's necessary to remove the lower rear seat belt anchor point bolt. On our car this was seized solid and I ended up shearing the head off the bolt. Looking at the problem I saw that the bolt tail pokes through into the rear wheel arch without any protection and was rusted solid into the captive nut. I managed to remove the remaining stud by drilling and using an eazy out. The bolt was very soft and drilled very easily. Checking with my local Ford dealer for a new bolt was a surprise. It's not available apart from as a complete seat belt set, £280!!!! Going to a local fastener dealer with the two parts of the bolt got me a cad plated identical bolt for £1.00. Still not found the source  of the water leak though.

Just to add to this. This is a common problem as documented on the Fiat 500 site.