If the light isn't coming on before cranking, I can think of several possibilities:
1)The light is burned out.
2)The 3-prong plug isn't connected to the alternator.
3)There is something disconnected between the light and the 3-prong plug.
4)There is something wrong with the light.
Check between the "I" terminal on the regulator (hopefully it's marked) and ground for 12V when the key is in the ON/RUN position.
The light is connected between 12V and the I terminal.
If there is no 12V at the I terminal, that's probably why your alternator isn't charging.
