Ignition woes

You have to follow the path. The terminals, connectors, switches in the path. The big big offenders are the terminals in the bulkhead connector, the ignition switch connector, THE SWITCH CONTACTS themselves, and less often, the ammeter terminals, and rarely, but has happened, a failure of the welded splice. This is taped up in the harness behind the cluster in the BLACK ammeter wire a few inches from the ammeter.

"Chase it." (the drop) Check where you can and make notes, maybe even draw out the diagram. See if you can probe it at the interior end of the bulkhead connector. If not, drop to the ignition switch connector. Check BOTH halves of the connector

I often recommend a "work around." If you can't or wont replace the switch (example) then cut the IGN1 wire coming out of the bulkhead into the engine bay, and add a relay. Use the cut end out of the firewall to trigger the relay, feed the relay from the start relay "big stud" and connect the other end of the cut wire to the switched load contact.