I tapped into the wiring for my reading lights up by the rearview mirror in the ceiling console to wire up my gps. (this circuit is always on)
I didn't really hardwire the gps though, I put a cigarette lighter socket right behind the lights in the garage door opener holder thing up there though so I can easily take the gps to someone else's car if I want.
I've hid the wire from the gps between where the celing console meets the headliner, you can only see about an inch of it going down into that garage door opener holder thing.
When I was a teenager I took a radar detector, pulled out the LEDs and speaker, I hooked them back up to the radar detectors board with long wires, then I kinda weatherized the radar detector with plastic and silicone and mounted it inside my front plastic bumper-cover with the LEDs and speaker inside the car. Radar detectors are legal here but not the greatest thing for a cop to see, he might have been ready to write a warning but as soon as he sees one, things change.
Might not work so great nowadays, back then, cops, at least where I live, couldn't radar you from behind like they can now and now they can laser you from front or back and a detector inside a bumper won't pick that up.
Is laser more common than radar these days? That show "Speeders" they're almost always using a handheld laser rather than a handheld radar or a car-mounted radar. I've heard that if you detect a laser with your detector you're already "got" anyway since there's not really a bunch of spread to a laser. i.e. If you're detecting one it's probably pointed at you.