I used to work at an upholstery shop and we would use a common fabric steamer to bring new life back to old furniture cushions. This works on almost all foam, simply run the steamer slowly over every avalable surface and it puffs back up like new. It works on car seats, but since the foam is rather thin, do not expect miracles. If you add a Monica or vinegar to get smells out, be careful to open all doors, for ventilation.
Not sure why used car dealers do not do this to broke down old seats, but maybe they do not know.
It doesnt have to be an expensive one either, mine came from Wallyworld, I think.

 

Just watched a video of this kind of restoration from AMMONYC channel on YouTube. He's a professional detailer hustling his stuff, but there are some take away things that he does that the regular guy can do with some basic supplies and equipment...like steaming padding to puff it back up! He also uses a steamer to clean leather seats. The results are impressive.