I finally had a chance to look at my car today. The ammeter wiring is exactly as described for the Rally Gauge wiring in the Q&A FAQ. Thanks for pointing that out, rohrt. I forget to check that area, and it is full of great information.
Anyway, the black/white wire connects to the power distribution splice, and the black wire connects to the junction block. I removed the gauge cluster and verified that there is 12 volts at both pins 11 and 12 of the cluster dash harness. I put an ohmmeter across the ammeter terminals, and it read 0 ohms. It is possible that the gauge is shorted out. I will have to look into it some more. I put my volt meter across pins 11 and 12 of the cluster harness and measured 40 mV with the headlights on, so there is a measurable voltage drop. It does make sense - there will be a voltage drop whenever current flows through a wire. The drop will be dependent on the current flow (amps) and the wire size and wire length (ohms).
I want to do some more testing of the ammeter gauge to determine what voltage drop causes a full scale reading, and put known currents through the power wire going across the radiator core support and measure the resulting voltage drop.
