Try to find a solution called Bac-A-Zap. Pest control people use it to deodorize dead bodies and it really works. I went to a pest supply store and bought a quart for about $19. It works instantly, as long as you actually hit whatever smells. I'd pour some in the ducts around the edges and be sure to use enough to get all the way to the bottom. You probably won't need more than a cup or cup and a half.

Mary