You are focused on the trunk lid, but if the biggerl problem is exhaust fumes, you need to look further - how are the fumes getting in the trunk,and then how are they getting into the cabin? It seems unlikely that they would be going from the tailpipe into the crack under the lid, and then into the cabin. Look under the rear seat for rust holes, and in front to see if what you are smelling is fumes from oil dripping on hot parts or something like that. The more common complaint about trunk odors is a raw gas smell.