Of course you haven't, and hopefully you'll never have to. This post on oppo last night, however, reminded us that it's something you could be faced with.
Don't want to read the linked post? I can't say I blame you. The quick and dirty details; it involved an unsuspecting Kia, a crash diet, and enough suger-free gummy bears to induce more liquidity than the fabled "brown note".
Now, maybe the poster was having a bit of a laugh, butt maybe not.
Having large breed dogs, and having the unfortunate experience of said large breed dogs getting sick on the way to the Vet's office, in the car poo clean up can be an unfortunate chore. It can be done, and it is as bad as you think.
Firstly, if it is really bad, like "cow on a flat rock" bad, you're probably going to have to remove/replace the area with the greatest damage. In other words, you'll probably need a new seat. Glove up, mask up, and remove the affected area. Once the brown washed seat is out, shop towel up any extra excitement you find and throw all that shit in the garbage.
Next thing, sanitize, and it's going to need to be more than just a wet-nap.
Get antiseptic/disinfectant wipes and wipe down every hard surface (make sure to test the wipe in an unnoticeable area to ensure it won't bleach your interior). If the BM was explosive, then bacteria will not just be contained to the drop zone. It goes air-born, like an apocalyptic poo-particle fallout. You're going to have to wipe all of the inside of your car.
After, use a carpet cleaner with a hand wand attachment. I can personally recommend the Bissell Proheat Pet.
Take out all the floor mats and start with cleaning the carpet (all the carpet), then move onto the floor mats. Let it air dry, then clean it all again (and again if there is any order left). Remember, if there is still oder, there is still fecal mater.
After this, sanitize again. When cleaning poo I follow Rule #4 from Zombieland; doubletap.
Once the car is all cleaned out, you can finally look at putting a new seat back in. Look to your local wreckers for a cheap replacement, they may have the same make/model that you can buy a seat out of. If you've got SRS airbags, you might have to clear some trouble codes if you switch the seat out (SRS probably won't work after though).
Now that I've finished writing this, I can't help butt think; "Crap, I just spent 20 minutes writing about poop." There you have it though, if you've got an animal or a child (or you eat a Komatsu 960e worth of gummy bears), and it pulls an "everybody poops" moment in your car, you've got a way of cleaning it. Other than "kill it with fire!"
So this article was obviously written in a jocking tone. So hopefully you got a laugh out of it. I have, sadly, had to clean doggie diarrhea out of the back of my hatchback. I did follow these steps, and it cleaned up without a trace. So hopefully you not only laughed, butt you learned something that you never need to use!