2 cameras on 1 head unit? how to wire?

What's the best way to go about hooking up multiple cameras to 1 head unit with only 1 av input?  I  want to have the one in the back come on when the vehicle goes into reverse, and then I want to be able to view any of them at any time.  What's the best way to go about this?  It will probably be only 1 camera in the back and one in the front.

What I had thought about doing was getting a RCA Y calbe with 1 male and 2 female ends and splitting the input between 2 cameras, then having the reverse camera triggered by the reverse lights and a switch that will simulate the reverse signal, and then another switch to trigger the front camera.  That way whichever camera has the switch flipped, it will show up on the unit.  Will this work or do I need to go about it some other way?

Let me know if this would work:

I've got 2 cameras that I'm wanting to view on one monitor (not at the same time though), one is the factory camera (rear) that runs off of 6V and the other is an aftermarket 12V camera (front).  I'm going to run a rca splitter off of the head unit so that both camera RCA's will be plugged in at once.  The way I will vary the image will be by cutting power to one and putting power on the other.

To do this I will have the rear camera on a relay powered by 6V and the front camera on another relay powered by 12V.  To trigger the relays, I'm going to use a standard 3-prong toggle switch.  I will run 12V to the center "accessory" prong, and then run the power out to the relays to each end of the switch.  This should run power to one camera when the switch is flipped in one direction, and then run power to the other camera when flipped in the other direction.

Am I thinking correctly here or would anyone else do it differently?  I've tried finding some sort of A/V source switcher small enough to be practical in a vehicle but I cannot.  If I could I would like to have more than just 2 cameras, and then I wouldn't have to rig up these goofy switches.