Does the starter relay 12v battery post need 12v?

That relay stud is commonly used for a "main" branch off terminal, but it also forms 1 of the two contacts for the start relay. The reason for that relay is that the Chrysler starter solenoid draws considerable current. That relay ALSO FORMS the neutral safety switch circuit for the automatic or clutch switch circuit for a stick shift.

You have 4 terminals. The two small "push on" flag terminals, one is start voltage from ignition switch, remaining one goes to ground through neutral start switch.

The big stud gets "major" 12V power, and the remaining "square screw" feeds to the solenoid. That one needs to be about a 12ga wire.