I don't have one of the newer units, mine is an ancient Garmin Streetpilot III (no spoken directions! Horrors!) about the size of small sub sandwich. I have it mounted directly above the cupholders, below the radio. I ran a hidden extension BNC cable and mounted the antenna on a suction cup directly behind my inspection sticker (barely visible from the outside of the car) and reception is outstanding.
Power is provided from an always-on, manually-switched three-outlet cig-lighter power pod that I hid under the glove box, next to the console. One of the outlets is for the GPS, the other for the XM Radio adapter (hidden away in the flip-out cubby left of the steering wheel), I could direct-wire the GPS, but not the XM, it's about 5v and would fry with direct wiring. Out of sight, out of mind, but I can recharge my phone or laptop if needed while still powering my other accessories. Never a blown fuse, never an issue. The original switched-with-ignition power outlet behind the GPS unit on the console remains untouched.
To mount it (and make it removable), I bent a flat aluminum piece into an acute angle and bolted the upright portion to the plastic piece below the radio (oh, stop screaming that I defaced the car, it's a plastic $40 panel that I can always get if I ever decide to sell it on Barrett-Jackson) for stability and to clear the cupholders, which I don't use for cups or beverages (I drive, not drink while driving) but I do keep coins and other small stuff there if I need to.
I will be updating it to a touch-screen model soon, and I have the Bluetooth capability in my aftermarket radio, so I'll post pictures then. But I hope you get the idea...