Another idea is to continue as you had planned except to use a phone or other mobile device for you to select content. You can then use Plex on a phone to "cast" the content to the screens. But then you don't really need the Pis - just hook the screens up to the server in the trunk and run PHT on the server. The downside is that the phone will have to be connected to the server but you can solve this by creating a network in your Car - a CAN (Car Area Network).