this is what i want:
(found somewhere)
Connected to Ignition
It is also possible to connect the camera or camera system to the vehicle’s ignition. Using this method, the camera will only come on after the engine has been started.
This method protects the vehicle’s battery from being drained, and is also a very popular ‘Always On’ camera method, used in commercial delivery vans and trucks.
The connection does not have to be directly at the ignition. Simply connecting to any device or cable, which is controlled by the ignition system, is sufficient.

just have to learn to which device and where is it.