What are you trying to wire? Is it in the car, or under the hood? How much power does it require? (amperage)
You can get a 12 test light and look at the fuse box. Turn the key in the on position and probe around until you find 12V. Turn the key off and see if power goes away. If it does, then you found 12V key on. If it stays 12V with the key off, move on to another one.