You'll probably just have to ask. The manufacturers put this safety feature in their units to cover any legal standings.
At my old shop most of the installs were done this way. If the model (and I am pretty sure Alpine is) has the capability to function while the car is being driven, the installer just has to move one wire usually. Its really not a big deal.
The units have a wire that is supposed to be hooked to the parking brake of the car. This makes the internal screen only function when the park brake is set. This same wire can usually be moved to a ground all the time, so the unit "thinks" the park brake is set and the screen will show the DVD being played. Most of these units have a "video out" as well. This output can be used for additional monitors (headrest, ceiling mount, etc) for the rear passengers to use. This output typically will work regardless of the position of the park brake. That way you can keep the screaming kids at bay while travelling. You can listen to the DVD but not watching it.
hope that helps...
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