On mine, the water was getting in the seal somewhere around the boot hinge, then running down the BACK of the liner BEHIND the rear seat bulkhead, emerging at the bottom (see white paper in picture) and then running down into the tub, but soaking the boot floor carpets on the way. Hence really hard to locate the source, since it was only wet on the floor and in the tub, not around any openings.
It was only found by my local body-shop locking an apprentice in the boot with a torch and methodically jet-washing the rear of the car.
Seems to be mostly fine with the replacement seal, although occasional dampness persists.
I then had the brake servo re-greased and the mounts replaced.
I also put a
water leak alarm in the tub (it was too loud when tested, so I dismantled it and packed the speaker with some blue-tak).