The fuse is doing its job by protecting the brake light wiring circuit which obviously has a dead short in it. A dead short is when the wire carrying 12V power to an electrical component is grounded unintentionally before the component. It's usually due to a wiring break, chafed wire or loose electrical connector/plug - anything that exposes copper wiring which can touch bare metal in the car body.
Common causes of shorts within the brake circuit would be a corroded bulb electrical connector, a bad bulb, water or condensation within the stop light housing or broken wire(s) in the wiring harness between body and trunk lid. Check all the brake lights including the center mounted light. Clean, repair or replace any suspect parts. Corrosion on copper wire appears as a pale greenish powder. Black soot or burns where the short is making electrical contact may be evident.
Until you repair the short, the circuit will continuously blow fuses. Do not be tempted to bridge the fuse receptacle with a piece of solid wire.