You need a bypass harness to plug into the back of your screen, not a case of cutting wires, Chris Rogers sells these, if you are lucky then you may have one installed by your previous jap owner, a give away is a small black oval switch located in cabin somewhere which turns the bypass off/on.
Not sure how the factory reverse system works, Chris could tell you, I run my reverse cam through my digital tv tuner, which has reverse cam input and switches automatically when reverse lights activate.