This mod will eliminate the infamous squeak you get from pulling on the seatbelt. The squeak is caused by the belt physically rubbing against a piece of plastic flashing on the seatbelt cover. The flashing is the result from being in a mold.
The black piece is what you want to remove.
From the rear door, remove the plastic plug.
You don't have to completely remove it, but the cover extends under the trim. Just yank up on it and it'll pop off.
The cover wraps around the B-pillar, so just yank and stretch to get it off.
Here's what it looks like completely unattached to the car.
This is the plastic flashing that is the culprit of the noise.
Many people will dremel it off, but it's much easier with a sharp knife. A box cutter would work well here.
That's it. That simple.