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that should be it
I am not refering to the firewall behind the engine , but the small one that separates the booster compartment from the engine.
That is the feed plug to the starter sol., so power that center wire with bat + and the starter should crank.. This test isolates which direction the problem is from that common point.
If Yes , then the problem is between ign sw and that firewall plug. The series circuit power chain back to ign switch from connector is K38 alarm
lock-out relay, to NSS , to ignition sw.
I have that schematic if you need tracing particulars

Also , see if you have 12v at ign sw 30 [red]