Each side light has two wires going to it. White wire seems to terminate at the ball. If I want to run a separate neutral/ground what would I hook it up to at the light end. Each light has two terminals - where would I attach the white?
The ground connection is the lamp housing. I'd put a ring terminal on your new ground wire and put the terminal between the head of the bolt and the housing.
Added: Automotive (trailer) wiring is suspect to corrosion. Get the shrink style ring terminals.