I would either use the switched 12v wire to the cigarette lighter for the red wires.

.

.

or a switched live into the stereo

and then the orange wire would go to the light on the cigarette lighter.

.

i would do the cigarette lighter cos its easy to get to.

.

.

and easy to tell which wires are which.

.

.

i dont know which colours are which.

.

.

but if you tell us what coulours you have there, then sum1 will come along and tell you which ones which