When I took out my interior i did replace the front seat foam, hell, it's an '83, it needed new foam.
The rear seats don't have replacement foam though. So I filled my bathtub with cool water, a capful of laundry detergent and about a cup of bleach and soaked the foam in it. I mushed them quite a bit, then rinsed them a couple times and hung them for a few days to dry. It worked.
My car had been sitting in a salvage yard for fifteen years. An entire rat colony had claimed it, so I do understand the smell you're talking about.
Maybe I'm not the swiftest guy, but I threw the covers in the washing machine with soap and bleach. They smell just great now.
All reassembled for over a year now and they smell great.
It's a bit of trouble, but it's as close to free as I could do.