7 pin plug on tow vehicle, how to test?

http://rv-mobile-services.ca/  The seven pin trailer plugs on travel trailers and RV's are problematic. In this video you learn to test them with a test light.

• #autos_vehicles
• #7_pin_trailer_wiring
• #7_pin_trailer_plug
• #trailer_wiring
• #7_wire
• #7_wire_trailer_wiring
• #7_way_rv_plug
• #seven_pin
• #seven_way
• #seven_way_connector
• #7_way_connector
• #seven_way_plug
• #7_way_plug
• #trailer_lights
• #travel_trailer_lights
• #7_pin_plug