Abstract: An apparatus is provided for electrically testing the standard light circuit of a trailer. The apparatus includes a battery and a flasher component electrically connected to a positive terminal of the battery. A switch is electrically connected to the flasher component. There is an indicator light associated with a respective trailer light. A positive conductor is interconnected between the switch and the indicator light. A ground conductor is interconnected between a negative terminal of the battery and the indicator light. A harness is releasably attached to the standard connector component of the trailer's light circuit. The harness electrically connects the positive conductor to the positive wire of the light circuit and likewise electrically connects the negative terminal of the battery to the ground wire of the trailer's light circuit. The switch is selectively opened to de-energize the positive conductor and closed to energize the positive conductor.