Dylansdad, welcome back? How'* the cute kid? Get a dry day, and use a garden hose to allow water to trickle over the rear window. After a little while it'll start trickling in the trunk back under the rear deck. If it does this, and you don't want to replace the seal, you can run a small bead of clear or black silicone sealant along the bottom edge of the seal where it meets the glass. I'm pretty sure it'* the glass side that leaks, but I'm not sure. Might try both.
Or you could just goop it and see if it ever gets wet again!