So I found these on ebay and had to jump on them even though they are the wrong color. I'd never seen or heard of these before. Apparently GM made a retrofit kit for the 85-88 cars that didn't come with 3pt belts in the rear. Now that I have kids and I'm getting my Tojan back on the road, I knew I had to jump on these even if I have to get the belts changed to black. Honestly, it doesn't look like it would be to hard to recreate
http://www.ebay.com/itm/201562978112

 

Yes, GM had those for several vehicles back then. You don't often see un-installed kits for sale, because most often people had the dealer install them. I bought a similarly complete kit to add cruise control to my '91 RS.

 

I believe all European imported cars came with those. Atleast mine did!