Does the rear hatch weather strip have a history of leaking air? I understand why my exhaust has the additional hydrocarbon, but I think its getting into the interior from an air leak in the hatch or something. Any problem areas I can check?
Have you tried adjusting the stops on the hatch? You miht also need to adjust the latch to keep it from rattling. Is the weatherstrip in good shape? Pretty cheap to replace.
Adjust the striker and stops to tighten up the hatch. Exhaust gases really should be finding their way in from the seal unless it's trashed. A new one from LRS is fairly cheap.
Do you have all the plugs in your spare tire well?
I have never removed any spare tire well plugs but I'll check to be sure. One thing is, when you attempt to close the hatch by slamming it down, it will bounce back up and not latch. In order to latch it you have to bring it down to the seal, then push down on the spoiler. It feels tight once its latched.