14 ga wire is fine. The wire should be connected to the same coil terminal as the wire that is going down to the distributor but a connection at the distributor terminal will also work.
The missing wire is actually a ground that starts at the solenoid. When solenoid plunger extends to engage OD an internal contact in the solenoid closes which puts ground on the wire. That ground goes up to the kickdown switch and connects to one of the top terminals. Those terminals are normally open so ground goes no farther.
When kickdown is desired, the quick accelerator movement to floorboard pushes the kickdown switch plunger and opens the bottom contacts in the switch so relay can drop out and cut power to the solenoid. Slightly after the bottom contacts open, the top contacts close to pass the ground on to the coil. The ground at the coil shorts out the points so spark stops being produced. The no spark condition causes the engine to miss and that relaxes the torque in the OD so spring in solenoid can retract the plunger and drop the unit out of OD. As soon as the solenoid plunger is released it starts to move back and the internal contacts supplying the ground to the coil wire open again. Ground goes away and coil can start producing a spark so engine resumes firing. The entire sequence is timed to last only a few milliseconds. There is not an exact time given for the sequence to occur and documentation says only that the action results in a "few" missed sparks.
No idea why someone would disconnect and remove the wire and switch but if the wire to the solenoid contact is shorted under the car or something is broken in the solenoid or something mechanical in the OD keeps the solenoid plunger from retracting quickly, the ground could remain present long enough to kill the engine. Keep that in mind if you have a problem after connecting things back up.
Here is a good 51-4 diagram that shows all the contacts.Attach file
51-54 R11 schematic.jpg (49.21 KB)
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