More than likely it's the aerial fitting. Mine does it too. I had a
look at it the other month, you have to take the alarm sensor circuit
board out to get at the inside of the aerial. Unfortunately it needs a
special tool or a bodge to tighten it as far as I can tell. You might
want to try smearing some sealant round it, I'll try it next time I
remember to buy some at the DIY shed. :-(

pull off the a pillar trim and check where its coming in
as a rule it should wet the roof lining a lot if its leaking in from the
sunroof(if you have one :-))
if its the aerial remove the interior light console and look for water there
(although usually with this the mast is either damaged or moves when you tug
on it)
if you think its the sunroof repost and i'll tell you how to seal it
if its the aerial just remove the nut pull it off clean and seal with
silicone sealant

If you have got a sunroof it could be the drains being blocked and
causing the channel to overflow.  You can test this by pouring some water
into the channel with the sunroof open and seeing if it drains.
This happened to me and I cleared it with a good spray of wd40 and a hoover!

Hope this helps

Konichiwa