Thanks all for the suggestions.
We have tried the perfume thing. The combination of dead and perfume is

smelling.
What ever died crawled down the air intake for the rear heater. I see no easy way to access this area of the car. If the smell does not go away, I'll have to start taking the rear inside paneling apart.
Last night I sprayed Lysol down the intake. I also bought some Ozium to use as a chaser after the Lysol. We will try this for a few rounds to see if we can make headway on the stink.
I had to laugh at my daughter this morning. She said our dog's breath smelled like the inside of Mom's car.