X 2. And like Bubba said in the other post, it's a good idea to run a ground from each light to the plug. My trailer lights were all individually grounded to the frame, then a ground wire was run to the trailer plug. It was a constant disaster. Driving on dirt roads can really screw up the ground through the hitch and will make your lights really dim.
The trailer has a ground wire that runs from the trailer frame to the plug and then from the plug to the frame of my car so the trailer hitch shouldn't matter. I'm thinking that the trailer ground wire might be corroded so I'm going to try and clean up where it connects to the trailer frame..