There is a hot wire grounding out someplace. You haven't replaced any tail light bulbs? If one is in the wrong way it will blow the fuse. If they are all right I would be looking at the brake light switch at the pedal. Disconnect the wiring connector to it and see if the fuse still blows. It could be hard to find the problem because there are so many wires and connectors on the circuit. Don't worry about a fire, that's what the fuse is for. Just don't put in a bigger fuse.