One some retractable seat belts there is a plastic switch that goes thru the door jam. Is this not on the 90's K5 blazers? The NOS seat belt below is missing this it appears but perhaps it has been redesigned from earlier versions.
http://i.ebayimg.com/images/g/c1YAAOSwFNZWuSrJ/s-l500.jpg
The plastic part can be seen in the upper right in the image below.
https://www.seatbeltplanet.com/sites/beams/uploads/images/Refurb/AFter_DSC06826.JPG
http://i.ebayimg.com/images/g/c1YAAOSwFNZWuSrJ/s-l500.jpg
The plastic part can be seen in the upper right in the image below.
https://www.seatbeltplanet.com/sites/beams/uploads/images/Refurb/AFter_DSC06826.JPG