I’m with Joseph.
With your owner’s manual should be a booklet outlining a process for following up problems beyond the dealer. Follow that. Document everything, requiring specific descriptions of the complaint on the shop order and specific responses from the dealer’s shop. Keep all of your copies. Also take photos of the exhaust residue. keep these as documneted evidence of the problem.
Also, contact the National Highway Traffic Safety Association (NHTSA). This is a safety issue.
A letter to the Atty General’s Consumer Protectioin Office can’t hurt either.
You can also have an independent shop put a “sniffer” by the openings and print the results. If exhaust is moving through those orafices, the sniffer will detect the exhaust components. That’ll be absolute evidence. Body cavities do not put out CO2, CO, and NOx by themselves. The cost should be minimal, less than $25.
Bottom line: you need to continue to push the issue, document everything, and use every outside agency you can to support your claim.