I put seat belts out of an 88 D-100 in the back of my truck. The seats I used were from a Nissan P/U. The seat belts had the retractor button that pulls the belts up when the door is opened. I built some caps to make them think the door is closed. Those buttons are not on the newer trucks, as I understand it.
To solve the upper connection, I welded an angle bracket on each side to the roll bar. You must understand that my truck is not really a RC and I really don't know where the roll bar would be in a RC. Another option would be to look for a Suzuki. The rear seats have a column with a bracket mounted that might be adapted to your truck. I will try to get a couple of pics a little later and show you what they look like.
First I found the Suzuki (The more I think about it, this was a Geo Tracker) with the mounts in it. I have no idea what year this is but it was one with an independent front suspension. Here are two pics of the mounts, one with the plastic cover and one without. I think if you click on the link below the pic you can get a pic that you can zoom some, at least on my computer.
And last, for the rear seat. Bear in mind that this is a home made body and the roll bar is made out of 1/4" thick square tubing. I used it to attach body panels to also so it's a little odd. Anyway, I attached a place for the top mount directly to the rear support. I don't know if anything like this would work in a RC because I have never had a RC with a roll bar.
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