There are two ways of looking at how you wire in an auxiliary outlet like this.
One way is how Luis did it, with the outlet dead when the key is "off", so that when you get out and walk away, you don't have to worry about whether anything is still "on". Peace of mind is a good thing. That is how I have my power point wired and when I use my old, tired-battery GPS it works fine when driving, but goes dead when ever I leave the car. I've learned to live with it, but it also can not be used as a place to attach a trickle charger. Keep that in mind if you store your car over the winter.
OTOH, you could wire it as "always on" with a fused line, and then the power point could be used as a charging point for a trickle charger. If you're not comfortable with that, most of the trickle chargers come with a selection of attachment accessories like a cigarette lighter plug, a pair of alligator clips and a 2-pin connector with pigtail leads. I installed one of the 2-pin connectors as an "always on" connection to provide an easy attachment for the charger. Best of both worlds.