Where the heck does it go? Is it switched? Is it a positive feed?

On the donor 79 truck it dissapears into the wire harness... in the 67 it's gone into it didn't have this wire ( or at least not this color and I have no idea what to hook it to. )

The Bat wire is self explanitory and the field wire(s) go to the regulator to be switched.

Help

Marc