That is for the sub woofer's power and audio. It is on all FJ's whether the sub is there or not, pre-wired for it. As for the rattling, i would say get a friend to ride with you (preferably a small friend that can move around the FJ easily) and can locate the sound to it's exact location, then go from there.I have a "metal plate clunking against another metal palte" sound when I turn left. I have read threads on the door latches, but it does not seem that they would sound like something loose hitting a metal plate. Or would they?
I removed the various plastic covers on the right rear interior storage box, looking for something loose. There is a large white plug facing the front, on the right inner wheel well. What is it for?