Ok now we're getting someplace.
That brown wire is the exciter wire that tells the regulator points to close and send current to the alternator to charge the fields and produce output. When you closed it manually that's what happened.
That wire provides the key on hot juice for the regulator and it comes from the key switch to the cab side of the firewall junction and connects with the brown wire on the engine side.
The cab side is a 24 gauge resistor wire which is connected to the accessory terminal along with a 12 gauge brown wire on the key switch. The resistor wire is the power for the regulator brown wire to close the points in the regulator.
NOTE: the 67 year key does not have an accessory terminal.
You may not be getting a connection between the two in the firewall junction or some one may have cut the resistor wire out of the harness not knowing what it is for.
Here is the key switch diagram. The black arrows show the resistor wire and the 12 gauge brown wire.
They continue to the inside of the cab junction (resistor wire) and to the fuse panel (12 gauge brown wire).
Fol;low the wire just below the brown wire on the key switch down to the lower left to the inside of the cab junction connector. If you pull the fuse panel you can see all this wiring. There are two 1/4 inch head screws diagonally in the corners of the fuse panel which hold the panel in place.
Here is a standalone of the wiring. It shows the wiring for a light gauge instead of the gauge dash with the ammeter. the gauge dash doesn't have the wiring for the light.
