I just cleaned 2 very dirty belts, actually 4 that will go in my back seat. They have been sitting for years and have decided to use them. Anyway, I used simple green. Sprayed them down on both sides and the dirt just started running off. They were soaked, i didn't have a brush or I would have used it. Just went about it by hand, than rinsed them off in water not getting the buckles wet. Where really dirty. I let them dry and they are very clean. One is slightly faded but the non faded one looked like new. Faded ones can be sprayed with a paint from SEM that can be used on fabric and will NOT rub off on clothes...
The plastic cover on buckle I rubbed out and polished. Not sure if yours are plastic or deluxe belts.
Or you can spend hundreds by the Pro's.