Can you hit the starter with a stick or something similar when putting 12 volt to where you got 6? If it works when you do that then starter solenoid is ng. You should have 12 volt on 6 volt wire when turning key. Time to get schematic and start testing.

Disconnect the wire with 6 volts on it and try jumping 12 v to that terminal. That will tell you if starter ok or not.

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Jim