Your diagnosis is pretty good; the '76 shows what you should see on the bue and brown wires to the ballast. The issue could be in the switch OR may simply be in the wiring from the switch to the ballast blue wire. Start at the blue terminal on the ballast and trace back through the firewall connection and on back to the switch. Or, check the switch in RUN and see if the test light is lit off of the blue wire from the ignition switch and if so, work back towards the ballast's blue wire. Note that this blue wire may pass through a junction or 2 on the firewall side; it feeds the ballast and ignition as well as the voltage regulator and one field terminal on the alternator.