Drivers air vent is a good place, if you only have 1 gauge that is. If you ever get more then I'd ditch the headunit for a carbon panel and install 2 or up to 3 gauges there. I personally hate obviously stuck on gauges on dashboards.
Cut the vent centre out, pretty easy, took about 30mins with a Dremel.
Haven't got a pic of it in the car as it's still not installed yet.
But if you go that route and install in the vent then;
For power a [mini] fuse tap in the fuse box, which is just left of the steering coloumn. Or, take an accessory feed from the headlight dimming panel.
For earth there is a factory point on the pillar behind the asc switch.