A bit late for the advice but I can guarentee it's your aerial base that is causing the problem, I had the same issue. you can either get a new aerial base or silicon the hell out of it so you have a seal to stop water getting in.
Pop your main interior light out using 2 flathead screwdriverd at each side. You will see one nut on a bolt that has an electric connection to it, undo this to take your aerial base off then do what you are goign to do and reverse the process.
ck
March 2009