A long time ago I had a problem with petrol fumes in the passenger compartment of my coupe. I traced the problem to a broken plastic 'T' fitting. I didn't trace the hose that it came from, but as there was no liquid fuel, just fumes. I assumed that it was a fuel tank vent hose. The 'T' was under the rear floor. On the right side. It's a small rubber hose of maybe 1/4 inch. Like a vacuum line.
This happened back in the late 70's, so my minds a bit fuzzy on the details.