If you put 12 volts to the S terminal on the starter it should turn over. The Purple starter wire runs from the fuse box area over to the back of the engine behind the distributor, and then into a protective metal tube on the back of the passenger side of the engine. It exits this tube near the starter and connects to the S terminal.
If you pull the air cleaner off and climb up into the engine compartment (done it many times, with one foot on each frame rail) you should be able to look at the back of the engine and see a large plastic T in the wiring loom just behind the distributor. You should also be able to see the wires running to the metal tube, and there should be at least three wires into the tube: (1) power from starter to fuse box, (2) charge wire from alternator down to starter, and (3) starter solenoid activation. The alternator wire is the easiest to follow since it runs from the alternator along the top of the valve cover and back to the tube.
I've also found it very useful to use a small mirror held behind the engine, along with a flashlight, to get a better view of the area behind the distributor.
The wiring is very near the exhaust, and subject to breakage if you remove either the engine or the trans. It also sometimes frays and gets grounded or cut on the metal tube. You can run a new wire through the tube, or you can temporarily run an external one just to see that it works.
You need to run some GM automobile wire to replace it, not the cheap wiring from the parts store. The insulation on the parts store wiring will not hold up well in the exhaust heat. It's probably only about 12 or 14 gauge wire, and I would look for the same wire in a junked vehicle and pull out a replacement section if yours is burned or cut.