If it's just running off the outside of the hatch, then I think it's a
design feature - particularly if you have the Aero spoiler :o(

If it's really getting in when the hatch is shut, then it might be the
"shark fin" mobile aerial. On mine, the aerial has perished and
rainwater runs around on the top of the headlining until it finds a
way down. I really must sort it out, but it only does it when it
really p1sses down.

Cheers,

Colin