Have you tried a visual inspection of the seals?
And makes sense that it would be a missing or broken seal as the fans would create a pressure and prevent the exhaust from entering.
Just remembered reading about vents behind the rear bumper that are used to relieve pressure from closing doors and such. Maybe one of them is stuck open? See if the smell is from one of the cut outs in the carpeting in the back of the trunk.
Edit: link to the thread
http://e84.xbimmers.com/forums/showthread.php?t=1339312