I mounted it high on the windshield just to the left of the rear view mirror. The only problem with this location is you can't see the visual signal if the visor is down.
The wires are easy to hide. You can tuck them in the headliner along the windshield, tuck under the trim at the top of the A-Pillar, then under the wide weatherstripping all the way down to the OBD. You can do it without any tools.