Hi trfcrx7,
I have a similar problem with water in the left wheel well. I searched the net and I came across this forum. I followed the tips mentioned here, but my problem was not a crack under the taillight- there was everything o.k. So I took the left plastic cover inside the trunk and examined thoroughly the area. I discovered that there is moisture around the extractor chamber (the black ventilating clap on the wall). I went on and removed the bolts of the left side of the rear bumper and there I discovered my problems.
First of all there was a crack in the side wall and second the gasket of the extractor chamber was not in good shape and it was not sitting properly against the wall. I wanted to be sure that the water was using one of the two ways to sneak into my trunk, so I wiped everything with a dry cloth and make sure, that there was not any water in the left wheel well. Then I took my sons water pistol and sprayed the crack on the wall. I turned out that decent amounts of water actually find its way into the wheel well. Then I wiped everything once again and did the same experiment with the extractor chamber. The result was the same. I was wandering what was the situation with the right wheel well, so I took off the bolts and looked inside- the wall was intact, and I guess thats why the right side of the trunk is dry.
So to fix the problem I sealed the crack in the left wall of the wheel well and reinforced the extractor chamber with a decent amount of silicone. I havent tested the result, because I only fixed it today, but I think that will do.
I hope you find my experience helpful.
P.S.
and some pictures