If it is plastic, as opposed to the seatbelt webbing material, you might try Mother's Back to Black if the plastic is a dark enough color.

As noted above, the seat belts do have an explosive pre-tensioner charge. They can be replaced if you know how to do it, but you really should get a detailed step-by-step instruction from the factory service manual. I believe the first step is disconnect the battery so that you don't blow the charge. Also, it will have to be a used seat belt as the dealers will *not* sell the seat belt directly to an owner.

k_enn