The diode across the coil is primarily for semiconductor circuits. When the coil is turned off, it is still charged(it's basically an inductor). You can get spikes in the circuit driving the coil.
In this case, you are using a switch to drive the coil, so you don't really need to worry about higher voltage across the switch. It doesn't hurt to have it if you want it. My opinion is that it complicates things.
I redrew the circuit to power both cameras at the same time. This will not power the cameras all the time; only when the switch is not in off or when the reverse wire is energized.
There are a couple ways that you can solder up this circuit. If you want 2 relays with a bunch of wire hanging off of them, you can buy 12 Volt relays cheap and get crimp connectors or whatever to attach.
You can also solder all of this to a PC board if you want a smaller circuit. I'm working on a project to use the cruise control lever to mute my radar detector. I am using this layout because I have other components to solder in and I want it to look clean.
Let me know when you've chosen a switch/layout. I should be able to add something if necessary.
I'm glad you/your wife appreciates my car. So many toys...not enough time.