I would recommend tapping into the rear quarter panel wire because it passes on the passenger side and you can easily find it. All you would have to do is remove one of the trim panels which come out pretty easily and then wire into it.
I'm using a Garmin mini and connected it to the auto-dimming mirror power with a dongar adapter. Highly recommend it, as it is easy to install and works perfectly when I start my bronco.
After taking a look at my bronco, I think the best spots for a mount are either going to be on the lightbar accessory area or the front bumper. I have the modular bumper so it should be pretty easy to find a mount that fits the screws. I looked everywhere on the back and there's really no mounts...
I just purchased two Garmin Dash Cam's. I plan on making a video on the install once they arrive next week. I strongly suggest not purchasing a dashcam from ford because they seem way too overpriced for what you get.
I'll link my video here. In it, I ran a wire to my auxiliary switch so I can use it as power for my subwoofer. I'll update this message with a new video once I'm done installing it. Let me know what other things you guys want to see on the bronco.