Update:

I pulled my RT-ENG voltage regulator out and tested it in a friend's '76 Fury. The LED came on no problem, as did his gauges. So I don't think it is a ground issue, I think it is a power issue. That would make a lot more sense at the moment. Per the '76 Electrical Manual, I've got three wires to chase:

* On the 6-point instrument panel bundle, E2 20 Orange
* On the 12-point instrument panel bundle, G1 20 Dark Green

I'm going to step out and run some multimeter tests, but for the moment, a question: the schematic is great for showing how the power from the instrument cluster voltage regulator is shipped out across the cluster (one line to the three gauges, one line to the instrument panel lamps, turn signals, hi-beam indicator, and one line that runs to the power source for the radio), but I'm not 100% clear how power is sent into the regulator. Any ideas?