It all started with a family members vehicle where the seatbelt wouldn't retract. I did everything I could think of to get it to retract, it would finally just go back very slowly but many times be hanging and you'd close the door on it. I went as far as removing the interior 1/4 trim to access the retractor and cleaned and lubed it with little results. This car was a low mileage clean car purchased from original owner. After googling over and over I came across one mention of cleaning the belts. So I sprayed some cleaner on them and wiped them down, no improvement. finally came across where someone soaked the belts to clean them and I tried it. The belts retract like the day the car was made! I've done this on evey one ofour vehicles and it made a big difference in all of them.
Pull the belts out all of the way and use a spring clamp to hold it out all of the way. If needed unbolt the pivot on the pillar. I use a small bucket with liquid laundry detergent and hot water and soak the belts. Be warned you will be grossed out with how fast the water turns brown from all of the dirt, even on a clean daily driver. On a real dirty one I'll change out the water and then rinse them in a bucket of clean water then leave them out to dry.

yep did it on mine. Big difference.

Something else that will gross you out. Cleaning the upholstery of a car that was smoked in.