Instrument lights non-op...

yah, that's correct, the individual bulbs ground to the metal part of the cluster which then grounds to the screw behind the cluster.

So, just as a summary, they stopped working again, but you pulled a wire and then all of a sudden they started working. But they only come on after you rev it? What doesn't work? the gauges themselves or the lights?

If its only the lights try checking to see if the contacts in the sockets are recieving voltage before you rev it. When doing this remember to put the negative lead to the ground (i always forget to ). If there's voltage then the problem is in the grounds or bulbs themselves. i'd then check the grounds, clean the contacts and what not.

If they're not receiving voltage, just as a formality check that fuse to see if its receiving voltage at that time. If there is voltage, then the problem is in the wiring, i think that node in which the wire splits to four of the connectors is going bad.

If the socket has voltage before you rev it when the others are off, there's no point looking elsewhere, the problem is in the ground or you have the wrong bulb size.

It's a process of elimination.
So if the socket end isn't getting voltage then the question is it the wire that has failed to carry the connection or is it the other end is failing to transmit the power. The immediate other end is the fuse block in this context. You then check the fuse responsible for the instrument lights, make sure you test each side of the fuse. it maybe just that the fuse is loose or there's some corrosion on the other side of the fuse.

If they're not getting voltage on either side then you go back one more step, which is the light switch. At this point you test the light switch if that I terminal is put out voltage or not, if it is, then its a wire problem between the switch and the fuse in the fuse block. If it isn't... then there's your problem right there in the switch itself.

If the fuse block is getting voltage on both sides of the fuse then the problem lies in between the socket and the fuse block which is some kind of physical wiring issue (totally possible with older wires) If that's the case then you can peel back some of the insulation to see exactly where the problem lies, if its before that node or if it the node itself thats the culprit.

If you're looking for an explanation as to why it works after you rev it, i'm no expert but i think that there's a small gap somewhere, loose connection or something, and when you rev it, it gives it just enough current to jump that gap and after the initial jump its fine.

From experience, i can tell you that in my 66's wires there was corrosion everywhere, i mean the wire ends were green almost, so its not common if that happens, and its a simple fix, just replace it.

I'm sorry if it was already mentioned, but I didn't see it. Did you try to turn the dimmer on the headlight switch and see if they start to work. I know mine don't work then the dimmer is on max brightness. For mine to work they have to be dimmed slightly. I have a new switch, but just haven't installed it yet.