Do you already have a backup camera installed?
I don't think you need relays, I would use a harness that is intended for backup camera operation to interface with the H/U and then put a Double Pole Double Throw(DPDT) switch on the outputs of that harness(i.e. switch power for the cameras on one pole and the camera outputs on the other)
You can tie the Video(-) leads together(tie together the 2 cameras and the H/U harness)
Here's a sketch(professionally drawn in MSPaint):
Here is the harness that I used for the connection of my backup camera:
http://www.autoharnesshouse.com/19867.html
It has a +12V output that could be passed through a switch as described.
Note that this will activate the selected camera when the reverse wire in the 28-pin harness sees +12V. How do you plan to trigger the Reverse signal?