Have you seen my Sharp 6" LCD page yet?
I have info there about which pins to connect video signal to. Pin 11 is the main Video Input pin, and this gets connected to the center pin of the RCA connector. The sheild on the RCA connector just gets connected to one of the GND pins (pin 6, 8, or 20). It would be a good idea to connect all grounds to a common point to reduce interference.
I believe you sent me an e-mail about this LCD a while ago -- if you did, I'm sorry I didn't reply sooner!
I hope you find this info useful, and I hope the installation goes well.