As another option, You could use an 'add-a-circuit' in a fuse box, too. Use a test light or multimeter to figure out which ones are live in a given ignition position. Those are good for 15 amp loads or less. From there you can run it down and out to where you need. The advantage here being that it'll have it's own fuse and that you won't have to expose any existing wires.
In my opinion, though, the absolute best way to add something is to take power directly from the positive post from the battery (either by eyelet or by an aftermarket battery terminal, like this one) and relay it to a switched power source.