I had a leak on mine and it was the devil to find. I systematically duct taped off the roof channels, rear quarter windows, rear window, chrome strip etc, sprayed with a garden hose, and still couldn't find it. Then I removed the cover on the tailgate hinge area and started blanking off there. Mine was working its way into the seam just under one of hinges and working it way into the cubby area.
If it was like mine, pull out the back seat and have a look underneath, to see if the water has got under there.
here is a post on it.
http://www.the75andztclub.co.uk/foru...d.php?t=251086