1987 Nissan Pulsar fuses...i think

Just 3 days ago my brakelights were working just fine. Now both of my tail lights arent working. What would cause the fuse for the brake light blow immediately after inserting a new fuse in it? I bought the #20 fuse according to the instructions, but I've gone through 4 new fuses, and only have 2 left. It instantaneously blows when I put it in. The car is completely off when I do this.

Thanks in advance for any advice.