to make it effective...you need:
[IGN +12V] --> [bulb] --> [alt]
If the alternator fails, it grounds itself turning on the light.
You need the 12V from an ignition source or you wont be able to turn off your engine with the key as the alternator will always be "excited"
This set up is for a new style 3-wire alternator.
check out
www.madelectrical.com for better info and diagrams.