Because these trucks are getting as old as they are, more than likely the cable(s) have stretched. This was a poor design to begin with. I'd say to pull the seat and get it onto a bench. There is a wire that runs from one side of the tracks to the other. It may have come undone or one of the wire ends has broken. There is also another wire that connects the side release to the tracks, so that hitting that, not only flips the backrest forward, but it also releases the tracks so that the seat slides forwrd. This is where the design is poor. This part can be a bear to work on. Don't know if GM still has any of those parts available, so the wreckers maybe the only solution. Problem with that is you could be buying the same problem.
When working on the tracks & the releases, be very careful. That big spring under there will slam the tracks from one end to the other so hard & fast that if your fingers are in the way, it will cause serious pain & damage! If the spring is under tension when on the bench, if you hit the release, the that 60lb seat will jump right off the bench!
One way to avoud that is to slide the seat all the way back in the truck, remove the front bolts, then slde the seat forward to get at the rear bolts. This will releases the tension on that spring, because it wants to pull the seat forward. Removing the seats this way also allows for the seat to be able to stand on it's own when on the bench, instead of you having to fight it when it wants to fall over. Also lessons the chance of that spring ramming the track to the rear..
