Mine used to leak like a waterfall and I could easily get 2-3 inches of water in my trunk within a few hours... which eventually cause mold and mildew.
Use some caulking to seal around the area you think is leaking.
Drill small holes to drain the water out the bottom of the trunk.
I don't recomend fixing it the way I did. My leak was caused because my chasis was out of alignment (the car had been wrecked before I got it) so I wrecked it again and the body shop straightened out the chasis.