STRAIGHT VINEGAR. Our cat got accidentally trapped inside our car all night and peed on my sons carseat. Just like you, I could get it all clean except the seatbelt which was undetachable from the carseat. I tried everything- soap, bleach, carpet cleaner, febreeze etc. As a last resort I tried the old grandma advice - straight vinegar. I just soaked it and let it air dry in the hot sun. It worked! It smelled like vinegar for a week or so but the small faded away nicely. No trace of cat pee or vinegar smell at all now. I would suggest putting a thick towel under the seatbelt(to protect your seats), drench the seatbelt with vinegar, suspend it with a bungee or something from your "hanger" attachment (the little hook people use for drycleaning) and leave your doors open and let it dry. They say vinegar can clean anything and I believe it! Good luck.