[RS3100]

If the carpets are dry, it suggests that the water is probably coming in to the boot area from above, judging by your comments on the seats. Dry the rear door and hatch serals if they are damp, dry out the boot floor with paper and/or rages to soak up the water. Remove the carpeted floor, spare wheel if you have one, tools etc. so the metal floor is bare. Once dry, sprinkle talcum powder over the floor, and smear it around the entire hatch rubber seal, and the rear door seals. Using a hose, or preferably a watering can with a sprinkler, pour water over the back of the car in different areas, checking the door and hatch seals, and the boot floor for signs of water which will be obvious if it has run through the talc. It will hopefully indicate where the water is getting in.

 

[RS3100]

The roof antenna is another good p[ossibility. I'm sceptical about rear light clusters or hatch drain holes, as the boot floor is lower then the rear seat bases. For the seats to become wet from a leak around the light clusters etc. into the boot, the water would then have to travel uphill to soak the seat cushions.