An intravehicular auto harness tester with switch consisting of a first housing unit mountable within or upon the dashboard of a motor vehicle containing three small light emitting diodes with wiring leading thereto and therefrom coupled with one resistor per diode, with such wiring entering the housing unit and leaving it by way of a coaxial cable leading to or from a second housing unit located typically in the trunk of the vehicle with three prongs and female socket pocket at a front end of the second housing unit for interfacing with an intravehicular auto harness power plug and with three female socket pockets and a prong at a rear end thereof for interfacing with a trailer harness plug and with wiring therein running from prongs to socket pockets and via junctions along such wiring also from the front end prongs to wiring in the coaxial cable leading to the diodes and a ground wire within the cable leading via an off/on switch from the diodes to the front end socket pocket of the second housing unit along with a ground wire within the second housing unit leading from the prong at the rear end thereof.