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Well because you can use a DVD headunit to power up rear screens... that's the point, anyways. It's strange, as a video dealer (if you're authorized) you have to sign a contract with the dealer, that basically says you will not install the units incorrectly, because all DVD headunits (or monitors) have a wire that needs to be connected to the parking brake, when the parking brake is set, the current goes through and switches the unit on... when the parking brake is down then the monitor loses power on that line and it is unable to display moving pictures (IE video inputs) it will still display the GUI however.
This is very easy to bypass if you just put a switch (paralleled with the brake) or if you just don't hook it into the brake all together. It's quite simple to just bypass, but then you couldn't prove to a cop that it's hooked up legally. It's strange some dealers (in the dealer contracts) won't let you sell video products over the counter. Everything has to be installed. It's really a good idea because mobile video misused is really quite dangerous!
Jeff
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