To save keep blowing fuses when trying to locate the fault I replace the fuse with a low wattage bulb. Remove the fuse, get a bulb holder and bulb now connect one side of the bulb to one side of the fuse holder and the other side of the bulb to the other side of the fuse holder. If it is shorting to earth then the bulb will light up. Now if you start nearest the fuse holder and move the tail light wiring about or disconnect parts of it the bulb will eventually go out which will give you a better idea where the fault is. If you are stuck for a bulb holder you can get a RX86T holder and a WL81C bulb from Maplins that will do the job.