Hi Ally1979,
See if the roof lining at the back window is wet next time it rains, if so then here is the fix.
You need to remove the black trim around the top and sides of the outside of the rear window.(Three pieces in total) Be very careful as the clips will break, you'll probably need to buy new clips. Once the trim has been removed, remove the metal bracket which is held on to the back of the roof with five screws. Once this is off you'll notice the white plastic grommets, remove these, clean them and the back of the roof. I put 'o' rings around them and window mastic (it's black gooey stuff that stays soft so it seals really well) Once you've done that you can put it back together.
Hope this helps.