First of all, are you talking, your trunk is wet or just the ledge of the trunk, right below the rear window?
If the trunk itself is wet, then Check if the whole ride side is uneven with the car. If it is, then you should be able to. If you don't want to replace the trunk(which I wouldnt do, just for a leak). I would replace the weather stripping, see if that cures the problem. If it does, good. then you are done. If not, there are other options:
1. Go to Menards, Home depot, etc. and find some addition weather stipping. Put it on top of the existing weather stripping and see how that works.
or(what I did) 2. Wash the car by hand and don't use pressure jets(lke car washes).
If the ledge is wet, it's not a big deal, every car gets wet there.