When I drive, my hands aren't always in the exact same position on the wheel. Muscle-memory won't reliably have my thumb landing on the correct signal direction. If I have to honk the horn, I need to be able to do that instantly, without thinking. Some cutesy icon located away from the edge of the steering wheel will guarantee that the horn sounds simultaneously with the "crunch" of another car backing into me.
My proposed rule-of-thumb: If a video game company wouldn't design their controllers this way, you shouldn't do it either for the most common—or most urgent—functions. Turn signals, wipers, horn, and hazard lights should all be real buttons that are in a consistent location. Horn should be in the hub of the steering wheel.