It may be worth doing a search on WC for this topic, I know I have posted my "setup".
I have one like this HD DVR 170° Car Dash Cam Recorder 1080p LED Night Vision G-sensor+Rear Camera
It has been wired so that as soon as the ignition is turned on the cameras record. I have had this permanently mounted for the past couple of years and it has worked fine, although I have not had need to use the footage as evidence, I have checked it periodically to see what has been recorded.
It records on a loop, so fit and hopefully not need.