I had given my daughter's cat a home when she moved and could not have pets. Autumn rolled around and I turned on the heat. That's when I discovered the cat had been peeing down one of the heating vents. It was overwhelming. I used Trisodium Phosphate (rubber gloves needed) to get rid of the odor, but it is caustic and I wouldn't use it on fabric. Have you tried Rhino spray? I'd test it on a scrap first.