10 gauge would work. Either I'm guessing it's goin' to your ignition switch or your fuse box. If the grey one is going to your starter it's gotta be the starter wire to the solenoid, which should be no different than a field wire to your alt. Aslong as it feeds 12 volts and excites it (engages the solenoid/washer).Make sure there is no diode in the system your replacing(sorry worked on boats, motorcycles and airplanes the last 2 years and have forgotten the systems). But should be pretty straight forward.

The Mopar Junky