wiring harness
Let me give you an example how I would approach this......
Hook your "big" test lamp from battery to ground so it is in series with ground
Hook up the engine bay harness, and make sure all wiring is "safe" from shorts to the vehicle ground.
Start with the "big" circuits, get your fuse link hooked up. Make sure EVERYTHING in the car is "off" door switches "off" (doors closed,), trunk and glove box lamp off, key off, etc,
Your test lamp should NOT light. If it does, stop right there!!! and find out why.
Continue with the "big" circuits........hook up the alternator output wire..........don't worry about harness dress/ layout yet, just hook it up. should have no light at your test lamp
"Rig" a test lamp to ground from the starter relay (yellow) "start" wire. Should go "hot" when you twist the key to "start." Try hooking this up to the start relay at proper place Hook up your neutral safety/ clutch safety switch. With the key twisted, the test lamp should now light "bright."
You can briefly hook your ground up "for real" and test, that the engine cranks.
Hook the test lamp back up
Let's say "down the list" you want to test your water temp gauge and oil sender gauge/ lamp If they are wired wrong, you don't want to fry something if the wire has "straight" 12V somehow. So you want your "big" test lamp in the ground lead again for safety
With the key in "run" both the oil sender and temp sender wires should show some voltage on a meter or light a small test lamp. This is because they go through the gauge "limiter" and will / should not be full 12V. With a test lamp from the temp sender to ground you should get "some" reading on the temp gauge. With a test lamp connected to the oil sender, you should get some reading on the oil gauge or the oil lamp should light "dimly." If this happens, these are checked and you can hook them up or mark them off
Just go on down the list. When you get to your brake warning switch wire, again, a small test lamp from the wire to ground should cause the warning lamp to light dimly, with key in run
When you get to the "blue" ignition run circuit, it should be "hot" ONLY with the key in "run." Should NOT be hot in "start."
And so on, one wire at a time. There just are not that many................