There is an enzyme you can use on the plywood under the pad.

Fellow landlord, you have my sympathy. I went through what you describe a couple of years ago. My cigarette smoking "cat lady" had lived there 19 years. However, we did not have any carpet. The kitchen and bathroom vinyl stank, but we were going to upgrade that anyway. The rest of the place has hardwood floors that refinished beautifully.

Oh.....[SIGH] yep.... When I moved, I left everything dust and smudge free with no scent. Today, after 10 years, a zoo smells 100 times better than the house. At least zoo animals do not smoke or make garbage. The tan carpet has been blacken..... oh well....

I have found that a blacklight + enzyme cleaner works very well. The blacklight will highlight the spots you need to clean and the enzyme cleaner will take care of it. I don't know what's available where you live, but you should be able to find something at most pet stores.

many things, including some cleaning products and detergents also glow under a black light.