Why do I get gas fumes smell in trunk and drivers compartment of my 91 Mazda Miata?

What you are smelling are the hyrdocarbons, which are the more damaging portions of gasoline to the environment. It may be a leak in the evaporative system, but being that you smell in in the rear of the car, it's probably the Charcoal Evap Cannister, or the hose leading to it.

The cannister is responsible for filtering out hydrocarbons from evaporating gas before it hits the air. If it's dead or not working this would be the fumes you'd smell. The cannister about the size of a Golf ball and is typically hanging at the top of the rear passenger fender well.