The bulb trick is to remove the fuse and wire a test bulb in it's place. When the current passes through the test bulb it will glow. In your case, with the license plate out of position and lights on, you will get see a dim glow in the test bulb and the liscense plate bulb. The reason they are both dim is that they do not have full 12 volts applied to them.

When you start seating the bulbs in the tailgate and the short occurs, the test bulb will glow brightly. You should the be able to make adjustments and find out where the short is located. You may need a helper to watch the test bulb while fiddling around with the sockets.

Choose a test bulb that is about the same wattage as the License plate bulbs. A much larger bulb might demand more current than the wiring can handle and a much smaller bulb might not show enough difference in brightness.

Dan