Look on the alternator for the IND marking on the terminal to use, the wire when disconnected with ignition on should be live, and when connected to the correct terminal on the alternator, with engine stopped ignition on should illuminate the bulb, so just check for a earth point on the alt, this will change to live as it starts to charge, the blue wire that goes to the instruments warning bulb is merely given a live one side when the ignition is turned on, the other side of the bulb is the blue wire going to the alt to a terminal that is earth(ground) when stopped but when running changes to live and so puts the bulb out as two lives are no good.
tabetha