My garage fitted mine and I wanted the same, they wired it off the fuel pump, not sure how correct that was but it works. I did subsequently read in the instructions that the oxford grips I have will only work off a certain voltage and that the engine has to be running and the alternator charging the battery to achieve that voltage, hense they can't be turned on by mistake.
Still prefer them being linked into the electrical system. I work at a Uni and I was worried students might fancy a laugh, leaving me stranded when it came to home time.