Getting rid of car smell. Smells like something crawled in there and died. ?

My boy manages a fast food restaurant. He tracked soy oil and chicken grease and other organics into the car. We've cleaned it and the inside is spotless (it does have some minor stains). The car itself has no leaks, body integrity is good and there has been nothing spilled. Interior is gray. It smells like something large crawled in there and died by pooping itself to death. It's not there. I've checked. Would value any suggestions that you might have. I do not want to take the carpet up