There's one place where it is very likely to be coming from. I had a leak in my '95 coupes trunk that I couldn't figure out where it was coming from for the longest time. Finally got sick of it and decided I'd tear apart the rear end and find the leak. Well, it wasn't long into "tearing" apart the trunk that I found the problem. There is a drain hose that runs from the roof, down the c-pillar, back into the side of the trunk and drains water out from behind the bumper cover. What happened was the adapter for the hose to the outside was clogged with dirt, grime leaves, and just nasty brown stuff. So the water was getting backed up and it would overflow and drain into my trunk.
Here is a picture I just got off google real quick. It's not an older gen so disregard that fact. The red line is where the tube runs from the roof to the trunk. The blue circle just shows where the general area is. All you have to do is pull the carpet cover from that side out, find the hose and the drain spot and clean it out if it's dirty.