I had this problem on my '95 88. I pulled all the carpet and drained the water. I knew it wasn't the trunk seal so I took a hose and with the water running slowley, I ran it over the rear window (no leaks) the antenna (no leaks) and finally around the tailights. I found one of them leaking like a submarine with a screen door. I removed both lamp assemblies, cleaned both gasket surfaces, laid a very thin bead of silicone sealer on the gaskets and reinstalled. Tested with the hose the next day and all was water tight.
I did have a leak problem on my Cavalier trunk which this process would no show any leaks so I figured it had to be the seal. The only way I found that one was to climb in the trunk with a flashlight and have the wife run the hose around the seal. I found the leak. The wife thought it would be funny to drive me around the block a couple of times before she let me out.