18 gauge wire from battery

Short answer -yes. IF that is the size coming out of the radio it would be fine to continue with that to a power pickup point. Even for 18, you are only going to have a fraction of an ohm if you ran the entire length of the car. That said, I usually run the next size up. If its got 18 coming out of the radio, I would go with 16. Fuse it at least to protect the 18 (like 5A), but could prob do lower 1-2 amp if its just a memory or even a backlight circuit. Your not likely to ever overdraw on a circuit like that, really doing it in case of a short. I wouldn't go to the battery, that gets sloppy looking.