Most Pumpkin DVD players have AV input and output functions, if your car radio also has AV output functions, everything is easy to solve.
If your car radio and your headrest player are connected via an AV cable, you can simultaneously watch the video on the car radio on the headrest player. Even if you need to use the car radio for navigation and other functions, the video can still be synchronized. Since your car radio can connect to WIFI or the Internet, you can also play online videos such as YouTube videos and synchronize them with your headrest player.
The simple operation brings more practical functional experience: