If your 10.4-inch screen only has 1 composite video input, you can use a video switcher to add multiple inputs. You can:
Build your own: http://www.elephantstaircase.com/wik...e=RCASwitchBox
Buy one from Radio Shack: http://www.radioshack.com/product/in...ductId=3070527
Or buy one suited more to what you're looking for: http://www.cartft.com/carpc/catalog/il/395
So, in your situation, you'd have the DVD player connected to your video splitter with 2 lines going to your two 5-inch headrest monitors and a third line to a switcher input. The backup camera goes to a switcher input. The switcher's output then connects to your 10.4-inch dash monitor. The switching between backup camera and DVD video only takes place on the front monitor.
Hope this helps.