You can install an android head unit, it has 2 video inputs, one for reverse camera ( auto activates when you put the car in reverse ) and one for a second camera that you activate yourself by opening an app. It's not perfect, but it works.
It is overall cheaper than retrofitting a BMW OEM reverse camera and you get a 8" or 10" screen upgrade and google maps / waze, whatever you like for navigation. This is what I currently have on my car.
Another option is to retrofit the 360 bird eye camera system from the 3 series, like shown here: