Had this on my ST, although it was getting wet in the boot. I adjusted the boot catch so it closes tighter, and also removed the rear lights and put some sealant round the two plastic bungs that the light locates into. Not had a problem since!

Also, I used to have a mk3 mondeo, that would leak through the rubber grommet that contained the wiring that went from the car to the bootlid. Water would leak through, onto the top of the headlining, run down behind the trim on the side and would soak the back seat. Took me ages to find it!