They used a vacuum switch to turn the light on and off. Ran off a 1/4" vacuum hose from the manifold. One side of the switch went to key on 12v and the other side of the switch spliced into the feed wire of the fender indicator. Supposedly, it warned you that the throttle was too far open for economy since the manifold vacuum was too low. It usually came on right about the time you could detect forward motion. Sold a lot of crap like this when gas hit 50 cents a gallon.
