I've had this in every dealers I've worked in various brands same problem same solution. Try pulling it all the way out and go at it with a vacuum cleaner if not go at it with a interior shampoo. Also thoroughly clean the guide on the b pillar. It's worse on black or grey belts because you don't realise how mucky they get. Particularly if you share the car with a girl as they tend to have residue of face creams and make up on their hands and neck that the belts pick up. Those are also what rotts cheap steering wheel leather making them melt/blister and peel.
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