My exhaust fumes (and rain) were entering through the tail light area.

So I put a set of halogen lights inside my trunk (power cord thru side marker light opening) and shut the trunk lid. In a dark garage, the light showed through all around the tail light assemblies!
So I removed the stock gaskets and went to my local hardware store and bought a couple of rolls of the 3/16" or 1/4" adhesive foam insulation. I recreated the tail light gaskets and the fumes (and rain) are gone.

Another area you may want to check is where the trans shift linkage comes up through the floor of the car. There should be a rubber boot there to help seal the hole in the floor.