How do I get DVD picture on screen?

Question

Have just brought a 2003 Honda Odyssey with what looks like a factory installed Necvox ceiling mounted screen. I suspect someone has subsequently changed the player head to an Alpine unit with an under seat 6 CD stacker. There is still power to screen and I can select some of the screen controls, but I am unable to get any picture. How do I remedy this? Is there any instruction manuals available?Thanks for helping, Bruce