I have a tin top so it was easier to find but was still confused how it could get in,then i found the rust on the top lip of the rear hatch seal,wasn`t very much but started digging and made a few holes,and i see now the way the internal panel above your head joins up with the seal housing metalwork,any little hole or split and the void above your head fills up when it rains heavily
Had to clean up the loose rust as best i could and bodge it up with loads of silicone,its working for now and hopefully last until i can fix it properly.