Sorry for the confusion. It's not the exhaust fumes. It's gas fumes from the fuel tank mounted in the trunk area. Even though the trunk area is sealed off, when filling the tank some fumes remain in the trunk. Eventually there will be an external filler. The trunk is well vented. There is an external vent ( outside of the trunk) on the tank too, but if the car just sits on a hot day the fumes will get into the car, this is what I'm really after.
The exhaust is out the rear past the body lip and does not leak.
The focus of the exercise is to vent the tank so fumes don't get into the car by the venting. There is an external vent but by capturing this I hope to clean up the car.