As its just an LED, if you want an ignition live, and reasonably easy access, you can tap into the back of the stereo, i wouldnt normally say this but as its an LED it wont do any harm at all.
Otherwise, you can probe the chunky wires in the back of the fuse box, i cant remember which one but one or more of those is switched live, easy to get too, just drop the box down off the clips.