These cars are horrendous for boot leaks really poor quality. I’ve just fixed my st for the same fault I had a few attempts then got pissed off with it and sealed everything I could find on google about them leaking. I removed the rear bumper and sealed the air vents and the plastic clips that the 2 8mm bumper bolts screw into. I removed the rear lights sealed the wire grommets and the 2 rubber grommets under the rear lights that do nothing. Also again sealed the plastic clips that the rear lights screw into. I then masked up either side of all the panel seams around the boot area and the boot hinges and sealed them. Also lifted the roof rails and sealed all the cracks I could see. I used tiger seal as my cars black luckily. Mine doesn’t leak anymore and we have had some very heavy rail. Also fitted a new boot seal