My leak was caused through lack of sealant/mastic near the back bumper on the left side. The bumper had to be removed to sort it, but it has been dry ever since it was fixed.
I was able to see where the water entered my boot, by removing the boot carpet and absolutely soaking the back area with the hose on the jet setting, whilst occasionally opening up the hatchback until I could see the trickling of water.
Next, I took a couple of digital pics to show the dealer so that they could check out that area. This helped them and a mechanic got in the back with the door shut and another guy gave the car a drenching so that they could see where the leak was.
Hope this helps you.