If your lucky. Depending on the tempature and moisture and fat content of your rodent you could have an even nastier mess in your vents. If its a rather large rat or other rodent and it liquifies you could be smelling the death smell for some time to come.

In a house where the tempature and humidity is more controlled it might be ok to wait for an animal to mumifiy. In an enclosed space such as a car its not such a great idea. The smell will make it into foam and cloth in the car and linger for a long time. No amount of scented pine trees hanging from your rear view mirror will save you at that point.

I would take an afternoon and explore the vents. You could remove the vents and use a small bullet video camera with infared light to explore the vents.. A long screw grabber might allow you to grab the corpse and pull it out of the vent without dissasembling your dash....