Oof... Touching a nerve here! ;-)
I used the generic GM Muscle Car Painless harness on my 70 Camaro, and I ran into a few issues. One specific to the topic at hand.
On the back-up lights, there were wires inside the car that were part of the wiring which went to the console area if I remember correctly. I routed through through my tunnel to my T56 and hooked them up. Then, when I connected my battery, I suddenly had the lights on. Huh?
Turns out that, at least on my harness, the backup light hot wire was ALWAYS hot. I ended up picking the power for that wire up somewhere else. So, you might check for that if you can before you complete all the wiring...
I wired the starter wire through the clutch pedal switch on my harness as well.
One other thing. The wiring for the power window circuits was woefulls inadequate! I added relays and my own wiring rather than using the Painless wiring.
I don't generally hear these kinds of complaints about these harnesses, so I don't know if I just got a bad one or what. But I know it caused me a good bit of frustration!
Good luck!
Bryan