If its anything remotely older its usually rear light seals. Of course I'm going to assume you've gone to the effort of identifying whether the boot seal itself is in good shape or not given it will have been right in front of you.
If its leaking so badly that you have a small swimming pool in the boot, the entry point/area should be fairly obvious and easily traceable - just run some water over the back for 30seconds, open the boot and you'll likely spot where its coming in.