Hi WaterDoc,
You could rig your system to activate only the camera desired by using the logic level change of the turn signal activated, using a comparator and a diode/capacitor as a peak hold, your system could hold the left or right camera on showing the side you're turning on. You time constant would have to be adjusted to hold the input high between each blinker pulse, each pulse would charge the capacitor, and the diode would prevent the return on the voltage (to the cap not the blinker lamp) to a low state, the cap would remain charged as long as the blinker was pulsing. Of course this input would have to be very high impedance, on the order of 470K or so to prevent the cap draining too fast. This could hook to the comparator + input, and a lower voltage say 6 volts to the - of the comparator, so that when the blinker was off the comparator output would be low and that camera would be turned off, and vice versa when the blinker was on for that side.
This would require some circuit design, but surely you're not the first dude to think of this, so Google may be your friend
. SouthArk was talking about the same thing more in a modular way though.
In Asia they have cameras with multiple angles, front bumper, back bumper, left, right, front corners, and even the most expensive ones have a fish eye lens on the top of the car and the system automatically stitches together all camera views so that in effect you're looking at the car from the top down. I've never seen this in person so I can't testify as to how good it looks in practice.
Good idea but I've found that glancing down at the dash during maneuvers in traffic is really dangerous. In Asia these cameras systems are typically used for navigating tight parking spaces and parking in general.
Good idea though!