When replacing a rear seatbelt, is it necessary to replace the pretensioner too?

The side panels to access the rear belts is fairly straighforward... The only trick is the narrow upholstered panels to the left and right of the rear seat backs. They have to be slid upwards with a good amount of force before they unhook and can be pulled off.

Once you get access to the seatbelt spool, I'm willing to bet a simple cleaning and inspection of all the mechanisms will resolve it... Usually these problems happen when the belt isn't lying flat at the start of the spool, or the spring that allows retracting is jammed up with lint and dirt.

 

I can't recall that I have ever had a seat belt dirty enough in one of my vehicles to merit washing it.

If you have it completely off the car and detached from anything mechanical, then a little laundry soap or dish soap in a bucket with hand agitation, then rinse in clean water a couple of times. Maybe a final rinse in distilled water ( but I doubt a little calcium residue on the fabric will hurt). Dry completely before reinstalling. I would avoid anything that might effect it structurally though, like acids (even vinegar), bases (products with ammonia), steam, or pressure washing.

If just one section is dirty pull the mesh out all the way, clamp it somehow (one of those kiddy seat clamps would work), and wash the dirty part.

If it is dirty in the mechanical part you will need to clean that too. Dry thoroughly. At that point some parts might need to be (re)greased.