Which I don't have right now :-( We're still looking at a location to fit one in a 4.8 LHD GU.....the short answer to which is there isn't an immediate solution that doesn't require re-location of other engine bay ancillaries :-(
Well, there is. But it's not advisable. Physically, you can tap into any constantly live circuit but you run the risk of overloading that circuit which is why I'd suggest running it straight to the battery through a fuse. It'd take 15mins (depending on skill level) to set up.
I don't mean to sound rude but if you know enough to be able to do this job, you should know all this.
Another idea is to buy a 2nd cig lighter socket & hook it up directly to the battery through a fuse. That way you don't need to modify your original wiring harness.