A simple trick is to replace the fuse with a pair of spade terminals, then use a pair of small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well in this application. The bulb will be full brightness when the circuit is powered up and the short is present so don't lay it on carpet or against plastic door panels. It will limit current to a safe value. Now you can disconnect things and move wiring harnesses around to see what makes the short go away. When it does, the bulb will go out or become dimmer. If the other tail lights try to turn on, they will not be full brightness because of the added resistance of the test bulb. The test bulb will still be fairly bright because it's trying to pass all the current needed by the other tail and running lights. You might have to watch for a very subtle change in brightness.