You probably already know this, but just to be on the safe side: Avoid acidic cleaning agents like bleach.

Also be careful to avoid situations that might expose the webbing to battery acid. It's very bad for nylon.

As others suggested, I wash my climbing ropes and other nylon soft gear with woolite. Were I to need to wash my seat belt webbing, that's how I'd do it.

https://www.cmcpro.com/what-happens-...ts-on-my-rope/

--Bob K.