have you heard of a test light?

 

Actually no I haven't. You see I just guessed at what wire the ignition switch was so I used this device that when powered will illuminate a source of light. I used this device to find out which wire would turn on this "light" when the ignition switch was engaged and off when disengaged. I would never have thought that it was refered to as a "Test Light" untill you poped up your question.