I bought this small laundry washing machine for about $10 on Temu. I used Bissell carpet cleaner w/warm water, it has sort of a slick feel when it dries and works great. But its a bit pricey! It looks goofy bit it pulled a TON of grime from the belts and they snap back much better!

 

U could save some money and just buy a bottle of concentrated water based degreaser, something like Purple Power. Get a smaller bucket or tub and mix up 1 gallon of water with 16oz of cleaner, or something appropriate for the condition of your belts, unwind them and then submerge the primary section of the belt that gets used and dirty and let it soak for a few hours or overnight. Save the cleaning solution for the other seat belts. Soak your clean belt in warm water for a few hours and then dry it with a lint free towel. Finally wait another hour for them to completely dry before letting it retract. The same solution should work for all of your seatbelts. This has worked great every time I have done it, no machine required.

 

i disconnect mine, and pull them out as far as they can go. i then hang them outside of the car, and use a pressure washer to spray the **** out of them. Let them dry for a day, and they will be good as new

 

I use the same detergent but remove the belts from the car, and do a rinse after the cleaning stage. They come out like new.