Given your previous description, no power on the tail lamp side of the fuse makes sense... you put in a fuse and it blows immediately. This indicates you have a short to ground somewhere in the tail lamp circuit. The blown fuse isolates the (shorted) tail lamp circuit from the power side of the fuse panel, protecting and preventing the wiring from burning up.
At this point, I would try putting the test light across the tail lamp fuse holder, with no fuse or a blown fuse installed. If the circuit is shorted to ground, the test light should light. (Power supplied from the fuse panel, ground supplied by the short.) Then try taking each of the bulb sockets out of the tail light housing and see if removing any one of the makes the test light go out. Also try disconnecting the license plate wire at the connector near the gas tank filler tube. Hopefully, this will at least allow you to isolate the problem.
