If you look underneath the rear of the car, in the rear bar, you'll see two small metal tubes, coming out on either side. Both have a rubber boot that keeps them from rubbing against the metal plates in the rear bar.
If you take your boot lining out you'll be able to see the two drainage pipes going down to the rear bar.
I must say, though, that one of the metal tubes (that connect to the plastic tubes from the sunroof) in my car was totally rusted to hell. I thought this was the problem and replaced it with a rubber hose of roughly the same size. I still get leaks though, so I'm going to rip out the roof lining and try to see if there are any other blockages in the drainage tubes.
As promised months ago, I will take some photos of what I am talking about tonight as I've got a digital camera that someone lent me for my fibreglass adventures.