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Odds are that if they are using RCAs they know you have a low voltage input (I think RCAs carry 1.5-6v compared to the 30v that automotive speakers peak out around). I suspect if the monitors have RCA in and 2.5mm out, that they have a tiny amplifier to boost the RCAs signal enough to hear them on head phones.
You should be able to get away with Y cables or adapters. But they do make audio/video selectors for cars with multiple inputs and outputs. I found several things on eBay by searching with "car video switcher" (leave the quotes out). So, passengers could choose between more than one DVD player, a game console or two, or a rear view camera.
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