[twinmill28]

I've got the monitors in the first picture. One was not working when I got the 2, so I traced the wires (they were tucked up along the underside of the console next to the drivers seat. There was a miniature connection next to the seat, kind of like mini-RCA jack, that had come undone. I reconnected and taped it, then followed the wires to a small black box zip tied at the firewall above the gas pedal. I don't remember what connections were there, I don't think RCA though.
I'm not so sure you could Y into those connections with any sort of plug to do what you are trying. Sounds intriguing though......