Brad, take a look at
this site for a wiring diagram for each of the different plug types. Check the trailer plug between the ground and each of the other contacts after you have removed the bulbs. If you get an Ohm reading of 0, you have a short in that circuit. The reason for removing the bulbs is that with the bulbs in, you are just reading the resistance of the light bulb rather than the circuit itself and it will be harder to detect a short circuit.