I'm having the same issues.
You need to find where it's coming in from first, that's half the battle. What I'd do is dry the boot very very well, remove the carpet, and interior trim at the rear, lock yourself in the boot, have a mate slowly pour water over the back with a hose or a bucket and try and find it that way.
Possible suspects at the boot are the tail light gaskets, boot lock, aerial mount (if aerial fitted to roof not wing).