I would look hardest at the harness at the rear of the car, especially (as noted) if trailer lights were added, or if there was collision repair (and pinched wires).
You might also remove all the bulbs, put a brick on the brake pedal to energize the circuit, and then re-install the bulbs one-by-one until the fuse blows. This might tell you if it's one particular socket/wire, or the entire circuit.
If you can get a schematic, you might see if there are any other modules or doodads attached to the brake light circuit - such as a brake light minder or a cruiser control kickout. A Corolla is less likely to have this stuff, but you never know.
Colo Springs, CO