He also failed to mention there needs to be 12V to the other side of the bulb.
As Cletis indicates, Switched 12V to one side of bulb. Other side of bulb to Alternator. I.E.- Motor off (IGN on): Alt is ground, bulb lights. Motor on: Alt generates 12V and equalizes both sides of bulb, bulb goes off. Alternator or charging circuit malfunction: Bulb glows weak or off/on.
A diode may be used in the IGN to Alt wire to prevent "run on". It is not necessary for the indicator bulb circuit operation. Remember it prevents flow in one direction and may cause the warning bulb not to operate properly. It would seem like it has to be between the Ign and bulb, not between bulb and Alt. I would not mess with a diode as Roy and Beltfed say. If you have "run on", then we'll talk.
As Deve says. Fusible link is a must. There are several big wires on our trucks which are not fused. Not good. Add fusible links 2 gauges smaller than wire. Usually right at the battery. (sized by gauge 1, 2, 3, 4 NOT 2, 4, 6, 8)
