It seems that the water was not coming in around the taillights but rather past the piece of plastic that surrounds the gas cap. I removed that and sealed it with silicone. Hopefully that will do the trick. If the water is only coming in on the same side as the gas cap it's likely that the gasket around the plastic piece which surrounds the gas cap is the culprit.

 

It rained today and no more water in the trunk!