Amazon: trailer wiring harness tester Amazon: trailer wiring harness tester. AutoSonic 7 Way Trailer Wiring Tester, 7 Pin Blade Plug with Red LED, 6 Function RV Connector Circuit Tester, Trailer Hitch Wiring Tester, RV Towing Wiring Tester, Weatherproof. 3.1 out of 5 stars 23. $8 $ 8. 99. Get it as soon as Mon, Sep 30. Wiring Harness Tester | etrailer Circuit tester plugs into your 4-way flat trailer wiring harness and identifies lighting problems. Labeled LED lights illuminate to show which functions are working properly.