Great link there showing exactly what i thought - splitting the RCA video signals.On the Menavrus site it does point you to specific instructions regarding the connection of the receiver to the Medianav here , the translated instructions leave a lot to be desired but you should be able get the general idea.
I will admit that this was the trickiest part of the install as the Medianav does not have a dedicated female RCA socket for video input.
What you need to do is to connect pins 10(+) & 22(-) on the LH connector of the Medianav to the RCA connector on the camera receiver and as the Medianav connector does not have any contacts built in to these sockets to match these pins you will need to fabricate them. How I did this was to cut a wired female RCA socket from my box of old connectors that I have accumulated over the years ( I think you can get them from Maplins or similar retailers) and get a couple of small connector sockets that you can get from internal PC USB motherboard connectors by cutting them out of the plastic casings and threading the wires through pins 10 & 22 of the Medianav socket. This may sound a bit daunting if you are not accustomed to fiddling with computer internals so if you have any reservations then I would suggest not attempting it. You will first need to enlarge the pin 10 & 22 holes in the Medianav socket to enable you to thread the wires through and pull the small metal sockets so they are flush with the end of the Medianav socket and then splice these wires to the ones on the stripped female RCA socket. It's worth mentioning that all the wires involved are extremely thin so you do need to take care. There is most likely an easier way of fitting these connectors into the socket by partially dismantling the socket but I was reluctant to do this in case all of the other wires/connectors came out and without a wiring diagram I would be flying blind.
Again I would reiterate that if you are not comfortable with either the firmware installation or the wiring then leave well alone and forget all about it.
However if, like me, you think, well if the unit has this capability and I can increase its functionality then lets give it a shot.
At the end of the day you're either going to have a much more versatile device for very little money , or you are going to screw it all up and be left with an expensive piece of junk .