You bolt on where you want really, although most of them are mounted above the door.
You can connect the negative into the body and then its just the case of getting a positive to it, idealy switched localy so you can switch it from the back.
If I was to wire one in, I would probally just take a live from the closest place, be it a live spare, or new run from the battery, then fuse it, feed it through a switch up into the light.