Mine (usually) turn on automatically when you start the car. But not always, as the power fluctuations when starting the engine sometimes cause them to crash.
I have one of these in the front:
https://streetguardian.info/sg9665gc...d-gps-dvr.html
And a generic A118C from Amazon in the back. The Street Guardian camera is basically the A118C with upgraded components (e.g. improved sensor for better night images).
If it's going to be used a lot at night, I'd look at good night images as the main feature, since the cheaper cameras won't do a particularly good job.