I took them off and soaked them in Laundry detergent for a few hours, I used a old toothbrush and lightly went back and forth on them.......They look brand new, even the buckles!
I maybe spent a hour on them actually doing work on them, I can't belive how nice they look. And the belts are really soft, and the buckles move really easy now. I used a little WD40 on/in the buckles after they dried.
The water in the bucket looked like mud, You wouldn't believe all the dirt.
It was a easy job, it took longer to get the back seat out....since I was too lazy to pull the arm rests off...LOL