Hey guys, I just went through this too. Went through three different add-a-circuits(pigtails) before finding the right one. I know this thread is buried but just summarizing the other replies and my own experience for any future readers.
This thread helped a lot. Thanks Roybot, your summary was very helpful for when I needed to hardwire my new Thinkware F770 2 channel dash cam. Prior to that I was researching dual camera dash cams and it came down to the
Thinkware F770 vs. the BlackVue DR650S. Although both dash cams have a continuous recording and parking mode and other similar features - I was sold on the F770 due to several things like the more covert design, less overall vehicle power consumption, and a better quality 1080P HD rear camera vs. the 720P HD rear camera on the DR650S. Inside the settings it also can be set to shut itself down should the vehicle’s battery voltage drop below a pre-set level.
I highly recommended the Thinkware F770 for anyone looking for a discreet fully featured 1080P front/rear dash cam and doesn't need a display screen. Instead, you can view it with WiFi connectivity to the camera from a smart phone.