I keep thinking about a dash cam but never got one yet. Like you I don't want the wires showing. As the power socket is so far back in the car its going to take a lot of work to hide them I'd think.
I don't think an action cam would be idea, Dashcams are designed for the job with motion sensors, night vision, GPS, the ability to save sections in an accident etc. I don't think an action cam can do that.