1968 with no power in cab

[VetteVet]

Have you solved your issue yet. I am posting this diagram which will identify
the wires you are describing so that everyone can follow along.







The firewall connector is located in the lower left corner of the diagram. It is the 12 gauge red one and we are looking at the inside of the cab. This is the main power wire to the cab and if we follow this wire up into the diagram we see that it connects to a junction with three other 12 gauge red wires. which send all the power to the cab items.
To the left of this junction in the middle of the left side of the diagram is the horn relay, which is where you connected your jumper and you got all the cab power functions by providing 12 volts to the junction. The other wires in the junction power the key switch, the fuse panel lower right, and the headlight switch to the right of the diagram.
The only conclusion can be that there is an open in the inside cab wire between the firewall connector and the wiring junction shown in the diagram.