OK, well he has several options, but they are all going to be +12v, although a fully charged battery is actually 13.2v and when charging is 15v, we say 12v as a nominal figure. He can take the supply from the battery positive terminal, the battery feed to the back of the fuse box, or as long as he is not going to be drawing too much drain from a always on accessory like the cigarette lighter. My personal preference would depend on the location but would be either a direct feed from the battery or the feed to the fuse box. In both cases an in-line fuse should be fitted in the wire to the remote locking device.