Hi
Nasty dirty light colored belts that are really really dirty, I have found that, rolling it out as much as you can, then hook it on to something outside the car, then really drench it with a good grime cleaning product. the one i use specially for belts is one sold in most stores here in spain, to get stains out of white clothes, and stains in general . dont ask me what it is, but it works great. for belts, and also on those mercedes round the doors, cloth liners.
After i drench in the belt i then scrubb it with a nail brush, both sides,
then i dip as much of it as i can in a bucket of very hot water, to rinse it. this usually brings the belts as good as youll ever get em. then i just dri it off with a micro fiber towel.
sometimes youll have to scrub once again with the nail brush.
it takes quiet some time to do, id say about 20-30 minutes per belt
but it does work.