if you want it to work only when ignition is on, run a good piece of thick wire from aux on ignition to live on cig lighter fit a in line fuse, and earth from cig lighter to a good earth on car. I say thick wire, because if you use thin wire, and in future sell car and next owner uses a cig lighter there is risk of a fire.
Did it have a ciggy lighter at one point? If theres an existing hole for a ciggy lighter, the wire should be just behind the panel, unless they removed the whole wire back to the fuse panel (unlikely). I would check the fuse panel and see which one is for the ciggy lighter and run the wire from there.
go to poundland and buy a multi outlet unit and then run direct to battery with an inline fuse in it that way you have phone charger/satnav/ipod/mp3 player and even better halfs phone charger lol see pic
ALWAYS REMEMBER BIG BROTHER IS WATCHING YOU:::::::: Have a nice day and an even better one tomorrow