You need to remove the carpet trim for the left side and then you will see the well behind the rear wheel and the bottom of the aerial. If the well has water in there is a rubber bung at the bottom which you can push out to allow the water to drain and leave out. It only needs to be in if you intend going through deep fords.
Mine leaks in the left hand corner of the boot and then runs into the well so now it drains away.
To remove the trim there is one 1/2 turn clip near the rear light clusters and one push in clip near the boot to cabin partition which is harder to pull out.