Yeah, had the same on mine!
Its the join between the rear light panel, the side panel and the roof.
Look at the gutter from the back on either side, follow it around to the side where the light panel joins the side just behind the rear side window. There should be a joint piece in the trim over the gutter. Under that is a length of tape covering the join. Under the gutter at this point is the join of the 3 panels - its leaking there.
I just pulled the tape off, dug all of the old sealant out and dried the thing off with the hair-drier, much to the neighbour's amusment, then filled it with silicone including lifting the gutter trim and putting it under that as well. I then cleaned all the crap from the gutter and ran a bead along it that was thicker in the area of the joint, tapering away to the front and back to stop water pooling.
Mine is a dark blue thing so I used black sealant and it doesnt show up untill you know its there and are about an inch away...........
No more leaks.
Well, not there anyways.............
