I have an aftermarket Kenwood unit that displays images and plays DVDs. Like many aftermarket manufacturers, it uses a simple mechanism to disable the display when the car is in motion. It requires the parking brake to be on to enable the DVD display. In reality, the parking brake is just creating a ground for the "disable" feature. If you just ground that "disable" wire when installing the head unit, the DVD display will work all the time. Some manufacturers are starting to make more sophisticated mechanisms that aren't as easily defeated. But someone always develops a relay or something to defeat it.
My guess is that the factory head unit is more integrated into the car's CAN bus network and can't be disabled this easily. It probably requires a software hack of some sort.
Don't bust my chops about distracted driving...I'm just posting this for informational purposes. I've never even tried the DVD feature on my head unit. Here's a video that describes the process in most aftermarket head units...
View my fuel log 2015 Mirage ES 1.2 manual: 51.4 mpg (US) ... 21.9 km/L ... 4.6 L/100 km ... 61.7 mpg (Imp)