Third wire out of a battery pack hasn't been used for close to 30, maybe 35 years.
There was a time where servo amplifiers used + and - 2.4 volts to run the motor. The "third wire" was essentially the center tap of a series wired 4-cell pack and the motor had either +2.4 volts applied to it to make it run one direction, and -2.4 volts applied to it to make it run the other direction.
Then integrated circuit servo amps came along and integrated into them were "H-bridge" drivers to control transistors used to run the motor. The H-bridge allowed the full 4.8 volts to be applied to the motor and could be driven to run the motor either way.
The third wire has remained in many battery packs and the "deluxe" switch harnesses for all these years, but has been left un-connected in the pack.
It connects to nothing, and doesn't need to be carried through connectors.
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