If it is not the wires, as you say, did you check the bulbs? Could be that the bulbs are burnt out or there is a bad connection in the bulb socket due to corrosion. Try placing a 12v auto flasher unit into the trailer plug. The 2 prongs go into the slots that are on either side of the flat place on the plug. This will cause all your outside lights to operate. If they don't, start working from there. Hope this helps.
Bob