Consider a battery discharge prevention device
When I installed my dash cam I also installed a battery discharge prevention device. I used the Power Magic Pro Plus by Blackview. I have it set so that the dash cam remains on for 4 hours after the vehicle is turned off. This gives me peace of mind in a parking lot etc. that if damage to my vehicle happens I will have it recorded. I have never had it shut down because of low battery power because it is only set to 4 hours, but it is designed to shut off power if the battery voltage drops too low. I paired this with a wifi enabled HD dashcam also from Blackview. Some newer cameras have this power management feature built in.
Back to your original question. I just used a circuit tester and found which circuits were always live and what ones were only live when the ignition was on. Since the unit had its own inline fuse, I tapped power before the fuse in the fuse box so that I would not potentially overload the fuse. The wire appears to be substantial enough to handle the potential extra draw.
To route the wires I tucked it into the headliner from the rear view mirror to the drivers windshield pillar. I removed the plastic trim, ran it straight down and it came out behind the fuse box. Use extreme caution with this method if your trim piece contains an airbag. I have an 04 so no side air bags. The only wire you see is the 4 or so inches that comes out of the headliner runs down the rear view mirror and plugs into the camera.
