If it is the interior you would notice it before you turn on the AC when you first get into the car. You could try activated charcoal tied in some old pantyhose and placed under the seat. You can get it at a pet store for cheap. It will absorb the odors. Swap it out every few weeks until the smell is gone. It may be that some one left the window down and the carpet or insulation has mold. That would be a more expensive fix, removal and replacement. I would try the cheap fixes first.