I washed my seat belts out in hot water and detergent. (My wife helped boil the water, since I don't admit knowing how)

I forget which soap I used, but I'm sure I described what I did a year or two ago, if you do a search. I dyed them with ritz? fabric dye. They looked really nice when I was finished. The stitched in tags didn't survive the process. A strict restoration, these tags would have to be removed first and sewed back in later on. My seat belt tags didn't match my car, since my car was not delivered with shoulder belts, so I didn't give that concern any attention.

You would be amazed how grey or green or faded original black seat belts will look when they are filthy. My original belts and eBay shoulder belts looked faded and they were very stiff. Body sweat, sunlight, cheesburger grease and such will make the original belts look hopeless when they can be restored. My belts were stiff and disgusting. Mine came out of the restoration process clean, flexible, and with a deep black look with an even appearance even before the dye process. The kettle of hot water I discarded from the cleaning looked like a disgusting brew of dark crud. After the restoration process, the belts were like new,and soft/supple and flexible.

I bought Camaro shoulder belts for my project. I would not drive any car without being properly strapped in.