l can not get the lights to work on a new Harbor Freight trailer.The right rear light works, but not the left rear nor the sidelights.The bulbs are good.Electricity is going to the non working lights.And oddly enough, if l ground the test light and contact the nuts holding the left non working rear light onto the trailer bracket, the test bulb lights up.l do not think the problem is the light because I've tried two.Is this a ground problem?Right now I amgrounding the harness white wire to a painted trailer surface per the instructions?If grounding is likely the problem, how can l make a better ground?Many thanks in advance.l gotta get this boat in the water at least once before the lakes freeze over!

undo the ground wire and sand some of the paint off then hook the ground wire back to bare metal.

Bad Ground

Believe it on not, pull the trailer around the block a couple times. Most trailers ground through the frame, and it grounds to the tow vehicle through the ball. The balls rust due to non-use so when you hook up the trailer and plug in the lights, they do not all work. When you drive a short distance the movement grounds the frame, and WALLA! Try it, it works!

As the right rear works, the problem is a bad left ground or a bad socket. Put the ground wire to bare metal. Still no? check the socket - sometimes the spring inside it is hung up and the contacts do not , well, contact.