We recommend placing it near the windshield's "A" pillar, next to the driver. ( see picture below)
This will mean its in easy reach when you need to press the button and is is in a perfect location to get maximum GPS signal from the satellites at all time. It's also not in direct line of site of the road, so you can reach it to press the button without taking your eyes off the road or distract your driving.
If you are in a location where sunlight can cause elevated temperatures beyond the unit's maximum temperature range then place it out of the sun but you also have to remember that it should not be out of reach.