[Paul F]

I can't get my Chariot trailer lights to ground unless hooked up to the hitch.

The ground wire had pulled loose on the trailer tongue. I've tried touching the ground wire that attaches to the tongue on bare aluminum on the trailer and the hitch ball itself and exposed metal on the truck.

The lights only come on when I drop the hitch on the ball. The ground wire is dark in color, so I assumed it was corroded. So I stripped it back farther and it's still the dark color.

Do you think I should wire on a new plug?

[Firestone]

Sounds like it. Might want to run some new wires as well. I am having all kinds of issues with my 2000 trailer wiring. I might just rewire the whole thing on the off season

[Hudson River Steve]

If you do rewire I highly recommend you use a marine grade wire. Marine grade wire is made of many strands of tinned copper wire whereas standard hook up wire (which is used by most trailer manufacturers) is bare copper.
The black corrosion you are seeing is due to plain copper. You can splay out the strands and scrape them with a knife or razor blade. Another option is to chemically treat it. I have tried a solution of vinegar and salt. Fill a Dixie cup with vinegar and then add table salt until it begins to precipitate on the bottom of the cup. Splay out the strands of wire and soak in the solution for 10 - 15 minutes occasionally stirring the wire in the cup to get full coverage. When the wire is bright copper again soak in a solution of water and baking soda to neutralize the vinegar solution. The scrapping method is much faster so I usually use it.