I am wiring both a front and rear camera on my father-in-law's truck.  I want the front view on by default and the rear camera to come on / be displayed with the reverse signal.  I browsed the diagrams and came up with the following scheme with a SPDT relay and I wanted to run it by this forum to see if you agree with my thinking.

30 - 12v+ (acc)
85 - Ground
87a - Front Cam +
87 - Rear Camera +
86 - Reverse Light Signal Wire

Thanks for your help!

-James