Most modern car's have the vent pipe routed from the tank to the filler neck and then to a sealed system incuding a carbon filter so as not to vent petrol fumes to the atmosphere, look down the fill neck and you will see a small pipe poking through the wall of the filler, another thing to consider is the venturi effect if the pipe is placed in the airflow to the rear as I learned the hard way after walking into a stream of Avgas leaking from a Cessna tank vent when the wind was blowing from behind the tail.
Spud's idea sounds excellent and if it keeps Mr IVA happy c'est la vie.
