Exhaust fumes in passenger compartment

I have a 1991 Crown Victoria which has a persistent leakage of
exhaust fumes into the passenger compartment - especially when stopped. I
detected a leak in the small diameter right hand crossover exhaust pipe and
replaced it - no joy! There was some relief when I replaced the pcv valve
but it didn't last long. There is no indication - visually or audibly of a
manifold or other exhaust system leak. It occurs at all Heat/ AC settings
but is least offensive on Defrost/Floor

there has to be something leaking in the exhaust system....hmmm

how is the shape of the body and floor?

Is there a body plug missing from the fire wall /smallhole ???