The last picture looks like the retractor is bolted to the rocker panel. Before I continue, both my cars are Dodges and I don't have any experience with Dusters, but I'm going on the assumption that Dodge and Plymouth closely aligned their safety belt design. In that case, the rear seat retractors mount under the package tray, behind the rear upper cushion. If you look at your package tray, either from above inside the car, or below from inside the trunk, there should be two slits, one on each side.
This is my Coronet, looking up at the package tray from the trunk.
I installed rear retracting belts in the Coronet just a couple months ago. I had to fab up a couple of mounts because I was using retractors out of a late model Charger. Regardless, I think the retractors in your car have to also mount under the package tray. The rocker arm is where the end of the belt mounts.
My friend Greg
@Kern Dog and I started a thread on FBBO, documenting our seat belt project. He has a 1970 Charger, with the mounting anchor also on the rocker. Prior to 1970, as in my cars, all mounting points were behind and underneath the rear cushion.
Here's a link to our thread, it's a bit involved because we had to do a lot of fabbing to get the late model retractors to work. But hone in on the pictures in posts 67 & 68 and you'll see how Greg installed his belts.
Rear shoulder belts in a 1970 Charger
I forgot to mention; I checked the package tray on the Dart in case I wanted to replicate the install, and it had the cutouts too.
