check for holes in the chassis beside the gas tank
almost every ae86 develops some rust holes on the inside of the quarter panels, right along side the muffler on the drivers side, and the gas tank on the passenger side.
You can either crawl under the car and have a look or pull the interior panels off and look for dirt and crap on the inside of the panels, if their are no holes it will be spotless. Usually the interior panels will do a pretty good job of keeping the fumes out of the cabin, sometimes they don't.
This is the only spot on my western chassis that needs a little work, I've had one side welded up, now I've got to do the otherside.
Neil