I haven't done it, but it should be pretty simple. Currently it is tied to the ignition circuit, just splice the power lead for the beast into one of the "always on" power leads under the steering column.
Pulling the center console out to get at the relevant wiring is pretty easy.
Or if you want to get a little bit crazier, put in a an auxilliary fuse box that is fed by constant on power from the battery, and punch into that. (Closer to what I have).