Remove the tail light bulb/bulbs and refuse. See if it holds. If it does relamp it, but make sure you are using the correct bulb. Check the socket and make sure all is right before installing the new bulb.
If the fuse still blows, you have a dead short in one or more circuits attached to that fuse. Get out the shop manual and check what is attached to that circuit. Look at all those components and the wire leading to them. There is likely wire, fixture, or a supplied device failure. Your goal would be to disconnect each component in a logical order until you find the defective device. This assumes that the wire harness to each device does not show some obvious damage. No bare or exposed wire. If the short is in a wire or harness there will usually be some associated melting or discoloration of the wire insulation.
If someone has done some recent wiring or electrical modification, I would suspect that first.
I would check under the fender in the tail light area and see if the wire harness is damaged there.
Hope this helps.