Something that most probably don't or won't think about is the gasket behind each tail light. Those get old and deteriorate and can let water leak in. Worth a look and probably an easy fix if that's the problem.
Everything here is a posiible cause. I too checked these things when I had my trunk leak. I eventually narrowed the problem down to the area around the hardtop latch. I made new gaskets for both latches then still had the problem. Upon further observation I noticed that the original factory seam sealer in the trunk gutters had become brittle and started to split. I couldn't even see the splits they were so small. After using a screw driver to strip away the old sealant and a trip to the paint supply store I resealed the problem areas and my leak was gone