another interesting finding is this site:
http://www.geocities.com/wonderer369/G35Nav.html
The guy set up a gex dvd player with RGB and somehow seperated the sync out, using the following method. Not sure if this would work on xbox:
"Using the one spool of Single shield audio wire from Radio shack, solder the pink wire that goes from the Display. Using the other spool, do the same to the pink sync wire that goes to the NAV. Route both wires to where the GEX Green and Red Connectors are. Connect the Nav Pink wire to the Red connector and the Display pink wire to the Green Connector.
Now you should have a total of 5 wires from the units infront to the GEX. You now have a total of 4 cables containing these wires. "