Some headrest DVD player packages have HDMI inputs and some do not. It's a feature that may or may not be important for you. Here's how you can use the HDMI input to determine if it's important.
With the HDMI input, you can connect external video sources that also use HDMI as the main connection. HDMI allows for a higher-resolution than RCA cables and so with HDMI you may have a higher resolution video.
One of the most exciting ways to use the HDMI input is to add a WiFi Dongle to the headrest DVD player. With a WiFi Dongle you can then connect wirelessly to
your headrest monitors to your smart phone or tablet as the video source. This allows for video streaming, game playing, audio books, educational apps and basically anything you can do on your smart device you can see on your headrest monitor as well via the WiFi Dongle.
TIP: Most WiFi Dongles require a power source as well, in most cases USB, and so when you use the HDMI Dongle on your headrest DVD player, you can also use the USB port on the headrest for power as well.
Note: HDMI is one direction only. With the HDMI as an input, it only works as an input and doesn't work as an output. For example, the headrest touch screen will not communicate back to the smart phone or tablet. Navigation will still be required to be done on the smart device.
Here's a link to WiFi Dongles that will work with your headrest DVD players.