[Morris]

One side of the Alternator light should go to the battery via ignition switch.....and 5he other side of the alternator light should go to the alternator.....

When key is turned on the light lights up because it has a potential of 12 volts on one side....when the alternator starts putting out 12 volts or more....then the light has 12 volts on both sides of the light and consequently zero potential & the light goes out.....