Easiest way to trouble-shoot a gas gauge?

OK, Jim, so I finally broke down and consulted the schematic in the service manual. The gas gauge gets its voltage supply from the voltage limiter mounted to the instrument printed circuit board. The instruments run on 5 volts because 12 volts would draw too much current and would burn out their meter coils. From the +5 volt output of the voltage limiter, the circuit runs through the meter coil and thence via the chassis wiring harness and firewall electrical connector to the sending unit on the tank, the other side of the sending unit being grounded. I am therefore betting that it must be either a bad voltage limiter or a bad connection at the firewall connector. Since the gauge quit working on both tanks simultaneously, I doubt that it would be bad sending units. I guess I'm going to have to pull the instrument panel. While I have the instrument panel out, I can also check the wiring and voltages at the tank selector switch.