Easiest way to trouble-shoot a gas gauge?

This isn't about an A-body; it's about my '78 Dodge D300 dually tow vehicle. It has dual gas tanks with the changeover switch on the dashboard. A couple of years ago the gas gauges quit working, so I went all through the wiring and replaced it and replaced one of the tank senders, and then the gauge worked fine on both tanks. Now it has stopped working completely again on both tanks, so whatever is wrong must be in the wiring that is common to both tanks; i.e., between the switch and the instrument panel. Would anybody have some tips for an easy way to troubleshoot it? Sure would appreciate it.