What I have gathered is that each speaker is out of phase with the others, its the head unit that corrects this so the sound is normal.

That sounds unlikely to me. I don't see how the "head unit" can "correct" phase.

What controls the phase of the speakers is the +/- connection of the speaker wires. If the speakers are out of phase, it merely means the speaker wires have been connected wrong. To correct faulty phase, just swap +/- on one of the speakers.

If your speakers are out of phase, you will hear a strange lack of bass with the balance control centered, but when you change the balance control far to the left or right the bass comes back.