Barely breathe?....... REALLY?..... I mean....REALLY? C'mon now..
That said, one of the leading causes of gas vapor smell in the cabin, aside from obvious holes somewhere on the body, is the charcoal canister. It's job is pretty much to eliminate all the vapors from the gas tank, and subsequently route those vapors back into the intake where they can be burned as part of the mixture. I'd investigate this area.
If your injectors were leaking, the car would run like garbage, so it's not that.
And since you say it becomes worse when the heater fan is on tells me that it is coming from the engin3 bay. That said, lots of times the aerodynamics of a vehicle can actually cause exhaust from the rear of the vehicle to find the easiest point of entry (ie: rust holes, rolled down windows, bad panel gaps, bad weatherstripping....etc...
Are you sure it's gas you smell, or is it exhaust fumes? I have a hard time believing that gasoline vapors made it noxious enough to "barely breathe" , exhaust gasses on the other hand, DEFINITELY.
Did your eyes hurt with the smell?
This post was edited by OldAndyAndTheSea on Jan 22 2014 07:48am