I have both front and rear dashcams that are hard wired for ignition on.
For the front, I used the garage door opener (homelink) as this is ignition powered, to initiate a relay. The power source is a thick 12? AWG wire powering the Sunroof. All of this wiring and relay is tucked into the space above the overhead console. The USB power cable is fed through the gap between the headliner and windshield. My radar detector also feeds off the same relay, BTW.
For the rear, I didn't use a relay because I'm plugged into an unused relay on the trunk relay/fuse panel. (Correction: on my 4.4i it is blank and unused. On my 4.6is, it is used and has a relay plugged in). The leftmost relay is ignition powered. I tapped into the 12V+ coming out of that (it's the top middle plug) and ran the USB cable up through the trunk opening under the weatherstripping to where the wires come out from the body to the trunk, then tucked it under the plastic trim and have it come out the middle and top of the rear glass.
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