First, you go take a look under your house and see what you can see. If there are no holes in the ductwork, and there should not be, how did the critter get in? If there are holes, they need to be sealed using the proper type of tape. While down there, you may discover the dead critter is not in your ductwork, but just under your home. That means you need to seal the openings around the pipes and vents where the smell is getting in, and you need to repair the belly panel with belly panel tape. If the critter is indeed in a duct, you may be able to remove it from that entrance point without making a new hole in the ductwork. Otherwise, be prepared to repair any holes you make unless you like heating and/or cooling the space under your home.

Thanks...husband found a very smelly dead mouse in the ac unit inside the house ...everything else checked out well and with the help of lots of peppermint essential oil the smell is gone!

So glad you got rid of the smell!