Andrew,
when I fitted the Alpine head unit it was, essentially, plug an play. As others have said, the 12v feed is in the connector block that doesn't have the speaker wires.
The additional power feed is for a "direct to battery" feed, as the Alpine unit I have loses its memory if the power is turned off using the main switch on the "computer box". MMC doesn't fit such a feed.
The Alpine unit should also have a power out line, I needed this for the powered DAB/FM splitter. With one of these the standard MMC audio fitting antenna works VERY well as a DAB antenna!
