I've got a similar setup.
My detector is clipped to the visor, I extended the cable across the headliner, down the pillar, across the bottom of the dash, behind my console to a 12v plug end. No wiring is seen except for the cig. plug.
I hardly ever use mine, but I can plug it into a lighter socket on the side of my console that's controlled by a switch.
Later on, I'll probably wire in a relay triggered by a key on/run 12v source so it'll come on whenever the truck's running.