I think I have a decomposed mouse in the drivers's seat somewhere in the upholstery of my 2003 Toyota Camry. Is there a way to get the seat apart to see if it's in there? Is this something I can do myself or do I need to take it somewhere? Thanks much--it reeks to high heaven.

Removing a car seat and it's upolstery takes a lot of time. One unusual tool MAY be required. Hog rings and hog ring pliers. If it's bucket seats, you just have to remove 4 bolts and undo a few connectors to get the seat out. If it has side airbags then you do need to disconnect the battery cable and wait about 5 minutes before undoing anything.

I do think it's highly unlikely that a mouse would get into the upolstery because of the tightness.

Smells, like mold/mildew are more likely due to the AC system or water that got on the carpet.

Renting an Ozone generator has been suggested in other threads for stubborn smells.