So your cat is stuck in your exhaust pipe? I would think that would smell worse than a simple fart smell. A couple of points for you. You must have a really small cat for it to crawl up in that opening. If you place a can of cat food outside your exhaust you just might get that critter out of there. Just don't run over it when you back out. Rotten cat food doesn't smell too good either. Don't park your car near any PETA rallies. They may think you are being unusually cruel to animals (I mean lets get real here who stuffs a cat up their exhaust pipe anyway?) I'd hate to see them graffiti up your XFire with cruelty to animals stickers.
Maybe the cat is farting because its so hot in there? I guess there are a million reasons for this smell but all the other posters here are wrong. I think you have to seriously consider my explanations and go with my fix.
Good luck!

Pat