Sounds like the brown wire should be 16ga and the brown wire with the white tracer should be 24ga and go to the ignition switch. From my schematic it all ties in with the parking lights, marker and taillights, for the brown wires feed juice to those lights once out of the underdash harness. Do your parking lights work okay? Oddly enough, I'm putting the dash wiring back in my '68 right now, and going through the same thing. Good luck!