for my Blackvue dashcam I tapped into the power for the mirror. That power does not stay on when parked, but I am not concerned with the parked vehicle function of the camera and I am not keen on the battery drain that would cause. Otherwise I think the best bet is to run a wire all the way to the BCM.
Blackvue is top rated, but I find that the picture quality is mediocre to poor. It doesn't seem to cope with moving objects (includes objects that are stationary but have relative movement due to the vehicle speed). I am rarely able to read other vehicle's license plates or street signs, day or night. Honestly the only major value to the high priced Blackvue that I see is that it is reliable. I have had 2 other inexpensive dash cams before and neither lasted a whole year. My Blackvue is still working well after 4.5 years.
Reference this picture of a large bear at night:
and this picture of a very close up deer, also at night:
Here was the view out of my windshield just after midnight. Travelling more-or-less at the 55MPH speed limit, exiting a turn. No contact. Note her several friends on the road shoulder ahead. FWIW: -1.1G braking according to the instrument panel accelerometer.
Note in these images how the road stripes are sharp, but the important details are blurry. This is symptomatic of bad motion compensation; a critical part of video compression that is computationally intensive.