Yr of car??? Color of wire??? Does the alt light work currently? The alt light gets power from the IGN switch, the ground side of the alt light bulb is a 16g brown that goes to reg. There is also a bypass wire, it is a 20g brown w/white stripe. It also get power from the IGN switch (directly) and joins the 16g brown at the firewall bulkhead inside the car. The purpose of the bypass wire is so the the charging system will still work if the alt light burns out.