edwin_lcw,
I just had this idea. Why not you pull down the back rest of your back seat. Then remove your boot board. Remove everything out from your boot, including the spare tyre, tool boxes, car jack, umbrella, tissue boxes, condoms, guns, grenades, C4 explosives, porn videos, etc. Empty out your entire boot. Then you sit inside the car, at the back seat and look at the boot. Of course, the boot have to be closed.
Get someone to spray water on the car, rear half. Then try to see where the water come in from. Then fix that area up.