You have 2 places to try to inject video as EM suggested, it may help you a lot to try it.
http://suzaku.live-evil.org/injection-points.jpg
But as shown on the SAMS, it expects 5v pk-pk at that amp, and composite is only just about 1v, but you should see something.
There is also that 13v potential to deal with, so be sure to feed through an appropriate film cap .01 (guess).
I have done this myself many times with good results.