Just use velcro attached to the dashboard
area next to the clock
Was I really going that fast?
Much more info from Garmin's web site www.garmin.com
The GTR allows a very shielded position for your GPS - out of the wind and rain but right in your line of sight.
If you have a later model of GPS such as the Quest, you may need to make up a small wooden block to screw the plastic holder to. This can then be velcro'd to the dash in the same way as the GPS III