pretty sure you rear seat should have three..pre-'92 I believe they were all lap belts. my '92 had should belts on the outside seat. You should be able to see the slits in the seat where the lap belts came through.
There should be 3 lap belts, one regular adjustable in the center and two retractable ones for the outside seats, the belt should come from the center and have the buckle at either edge of the seat, but yes three in total.
If you don't have them I'm sure you can pull a set from a junkyard, or since I'm guessing your kids will be sitting back there, get them new from LMC, NPD, JBG, etc.
Three belts eh? Well that is reassuring. Truck left on its way to Michigan before I could ask the PO to check back there. He wasnt sure on the phone. Nobody ever sat back there. Here is another pic. It is just hard to tell where the other two mounting points would be located. Hopefully nobody removed them but as stated, could always pick up two sets from the bone yard or NPD. But from the pics I just dont see three belts. Time will tell.
Thank you for the picture. That is exactly what I had drawn up in my mind. Looks like I will need to find at least one other set of belts. For a truck in such good condition I am surprised that the belts are all jacked up.