If you use a tap a fuse on the glove box light circuit you can have always on 12v. The glove box light circuit is always "on". For export cars it is fuse 47. Not sure of US cars however it should be obvious if you look at the manual on the 17" screen fuse locations. If you want a circuit that is only on when the car is on then I suggest tapping into the 12v power supply circuit.
You will probably need to cut out a small portion out of the side of the fuse cover for the wire to come out. Please make sure you fuse it properly.
See link below for an example of the tap a fuse. They are available from auto parts stores as well.