I doubt it is possible without tearing it apart....dont know a lot about screens in cars but most of them I would think have all of the screen's wiring inside its case rather than in the wiring harness
even if you are able to find the main wires for the screen I doubt they are conventional RCA wiring
but sub'n to find out more info as I may be getting a speed3/speed6 as my next possible ride this year...
there is a way but as said above, it is a little pricey. My buddy did this to his, and it works just fine under his "Media" button. Here is the link to check it out.