Not sure what is meant by upper and lower wires, I just can't picture it. If the starter cranks when putting 12V to the "middle pin" the starter is ok. It sounds more likely to be a connection or a bad ignition switch. Check and clean all fuses for good measure.

What happens if 12V is jumped to the wire that is getting 6V in the crank mode? Nothing should be 6V, there is a bad connection somewhere causing the voltage drop and it may be inside the ignition switch or the neutral safety switch.

Have you tried moving the shifter to neutral and retrying?

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