I have a 68 convertible with Ralley gauges and also have a two year electronic degree. I worked in San Antonio a few weeks ago for 8 day too:)
Anyway, I can't seem to find any of my wiring notes, but from memory none of the factory wiring diagrams available will show a rally gauge setup, however many of the colors will be the same.
My car did not come factory with rally gauges. I bought a set off ebay and bought all new wiring harness from M&H for my whole car and a new printed circuit board.. Even with new wires I had a bunch of issues with my gauges. Mostly because I painted the metal gauge pod and none of the grounds worked.
Since it sounds like you have power at the plug I would new focus on the gauges. What I did with mine was trace out the printed circuit on paper along with what gauge it went to. I also used a 9 volt batter, a 100 ohm pot and some alligator clips to test the gas, oil gauge and lights. DO NOT USE IT TO TEST THE AMP METER.
Tracing out the circuit on paper will make the issue clear of what might be going on.
If I remember correctly the 12V feed everything. The functions are based on grounding. So for example 12V goes out to the gas tank sending unit on the Tan wire through the 90 ohm wire resister and then grounded.
Sounds to me like you may have a damaged printed circuit board.
