If your brake lights/signal lights work I doubt that it could be a ground problem as they all use the same ground. If you jump form 3 to 4 on the trailer plug, like someone said the lights should come on. If not problem is in the trailer circuit and could be a damaged plug. If lights do come on, then use a test light from 3 to 4 on the truck plug so see if you have current there.
If you can not still get it to work, try this. take apart one of the running lights near the front of the trailer, the ground should be white, so go to the other wire which should be hot. Connect from battery + on trailer to hot wire and see if lights come on. If they do problem will be in connector or circuit between lights and connector.