Our DIY system from 5 years ago is described
here. Two side cameras, a rear camera, and a huge monitor in lieu of rear view mirror. My husband wired it such that the view switches automatically to left or right when the turn signal is actuated.
We really want to upgrade this system to include a DVR and better cameras for security, but a vehicle-ready solution is not yet on the market, and it's too far down my priority list to design and source fully right now.
This is a cell phone video of the monitor - video of a video - so the quality is not great. The side camera itself is quite good, however.