dual backup camera wiring

Hi I am new here and need some advice on wiring a second backup camera into an existing system. Truck has an Audiovox backup camera system with monitor in the rear view mirror.  The four wires running to the camera are red, black, white, and yellow with what appears to be a shielding wire wrapped around it. Camera has audio besides video and connector only has 4 pins so I dont think the shielding wire is connected. I have a universal second camera mounted on the back of a 5th wheel. It has the same 4 color wiring as the front camera.  I spliced the second camera into the wiring as follows: black, white, & yellow all spliced into eachother. The red power wire out of the monitor to turn on the camera I ran to a relay with a switch so that the switch in one direction powers one camera and in the opposite direstion powers the other camera. After splicing in the wires  this way neither camera works. The switching is working because in one direction it is just a snowy screen and the other direction a line runs down the screen.  Sorry this is so long but trying to give all the information. Thanks... Andrew

You have to switch all the connections (except ground) - not just the power. (Unless you include some device which goes hi-impedance when unpowered.)

You are effectively paralleling electronic signal circuits, and the output of one (ie 75 ohm 1.5Vp-p) could be blowing the other, but is certainly stuffing up the video signal.

Thanks for the reply. So if I were to relay the red, white, yellow wires do you think this would solve my problem? I could isolate the ground also unless you think it is overkill to run a 4th relay.  Thanks again... Andrew

It should work, but you can't just switch connections - video signals often require impedance matching etc.
And not 3 or 4 relays....

You'd be better getting a 3 or 4 pole changeover relay (4PDT).

The ground may not have to be switched, but usually it is safer unless you know the power ground and signal ground(s) relationship (though often they are not a problem).
And maybe the audio isn't that critical - or could be joined via resistors...
But 4PDT should be enough.... ($10-$20 with base).

You might get interference, and the screen may "jump" as it switches between the 2 cameras.
There are other possibilities - like using a chip (CD4066??) to do the switching, or using KVM switches, but I assume you want a cheap "mechanical" solution.

There was a thread on something similar (switching between front & rear cameras).....

Thanks again. I will likely try the changover relay you suggested. I won't likely be switching back and forth, just using the truck most of the time and switching to the trailer when it is connected. So if I switch them while not in reverse it sounds like this will work for me. I was just looking for a cleaner way to install it and not have to unplug one under the truck to plug in the other when using the camper. I did try a search prior to posting this and didn't come up with anything.  Thanks...