How do i find and fix water leak (Renault clio dynamique Yreg 2001)?

In the back seats of my car when i lift them up there is a sort of metal troph which the bottom of the seats sit in.

In this troph, whenever it rains a build up of water collects. until yesterday which was heavy rain the water seemed to collect mainly when i was driving, except with this heavy rain i had a look in the morning and water had got in.

I dont think it is my sunroof as surely the leak wouldnt appear just in the back so i have a fealing it is one of the rear windows or boot or even a hole in the wheel arch.

How do i find and fix this problem?

James
June 2010