Help me my Eyes are watering

If you have air leaks into the car at the rear, it matters not that the exhaust does not leak. When you are tootin' down the road, air leaks into the interior by sucking air from behind the car INTO the car. Anyone that's ever owned a wagon knows this

You need a good trunk seal, and if the package tray is tight, consider at least plastic over the rear seat opening. Safer, of course is a metal firewall.

It helps if you can retain a good positive airflow / pressure through the car, IE leave the vent/ heater open. I unhook my heater in the summer, and almost never shut the cowl vents.

(I don't have this problem, it's to keep me cool, LOL)