Test Light "Look Ma! No Ground Clip!

That is not an AC proximity tester; it is a 12vdc test light like the type that normally use an alligator clip to ground. It actually uses your body as a ground path (hence the metal housing) and will only work if you are touching a grounded section of the vehicle you are testing the circuit on. I built one of these about 15 years ago using plans out of Popular Electronics. It uses a MOSFET transistor as a switch to turn on the bulb/buzzer (hence the battery); Although your body does not have the conductivity to act as a ground for a typical 12vdc circuit, it DOES have enough to act as the ground for the MOSFET.