It took us over a year to get the smell out of my Wife's car after it sat in there for a month while we were on vacation. Even took it to one of those fumigating places. Coffee grounds were suggested, Fresh Wave and we tried other things suggested online. Mostly time allowed it to go away. Do a Google search.
If the carcass is gone, bleach.... straight bleach ... and lots of it. Pour it on and just let it sit. After it dries up, then wipe of any residue. Been there, done that!