Hi this is a common fault with this model my 12 plate done the same thing it fills up around the spare wheel. It comes in from the side into the boot area it is a simple fix. If you have a look on utube boot leak on Ford focus mk3 it shows you on there it's a matter of removing the rear bumper and resealing the 4 vents behind the bumper it took me just over an hour to do it but I didn't rush. And ya only need a few simple tools and a tube of sealant. Their are some other comments on here back in around 2015 same problem hope this helps