The cruise control on GM vehicles has remained basically the same for a long, long time. Don't know the exact history, but I have messed with GM cruise controls from the early 80's through the early 2000's, and they're pretty much the same setup.
And by far the most common problem that I've seen (likely the cause of your problems) is broken wires. If you'll look closely on the back side of blinker lever, you'll see a black wire (actually a plastic-covered bundle of tiny color-coded wires, not unlike what's in a telephone cord). As the years go by and you work that blinker switch, the wires flex and eventually break.
There's a connector under the dash, and you can check the switches and wiring with an ohm meter. I don't remember exactly how it's set up, but I figured it out without a manual when I was 18, so it can't be that hard. If you can find ANY GM factory manual from 1980-ish on up, it'll probably have a diagram.
It's kindof a pain to pull the wires out and check them, but here's how you do it: On most GM's, the blinker stalk will snap out. Pull on it firmly and STRAIGHT. It'll be difficult, but it'll come out- be sure you don't yank the cruise control wires right out. There isn't much slack.
Remove whatever panel is necessary to see the wiring under the dash, push & pull & wiggle the cruise wires so you can find where it comes out of the steering column, and you should find a small connector down there. Unplug it and tie a wire or heavy string or something to it so that, and pull the cruise wiring up out of the column. That string will be use later to fish the cruise wiring back through the column.
(note: I've run into one application over the years that had a connector INSIDE the steering column- it was a late 80's Olds Cutlass Cierra, I think. Every other GM I've worked on besides this one had wiring that ran completely down through the steering column, so I'm guessing that that's how your S-15 is set up.)
Now carefully cut the plastic cover off the wiring where it flexes, and chances are you'll see one or two obviously broken wires. Takes some creativity, but wires this small can be soldered and taped up, or you can get creative with some shrink-wrap and make it look good.
