You just need to find a constant live wire to keep the memory going in the head unit, and a live wire that is active when the key is in the ACC position, so the radio turns off when the car is turned off. Get a test light and test the back of the fuse box to see which fuses do these two things. You don't need to worry about the fuses in the fuse box because the head unit should have its own fuse in the power wire in its loom.