That is usually the feed wire shorting against the body or other metal part, you will need to check for a bared wire somewhere in the looms. Its usually around an area you have been working or moved something about as they tape them up pretty good in the areas it can happen.
If you remove the bulbs from the rear lights it will isolate half of the circuit but the wires run in the same loom so it wont be that much help to finding the bared part. But if its on the return half of the circuit then it wont blow the fuse and you will know the fault is on the rear light circuit.