The one connected to the bundle for your 12v plug is for the smokers kit which gives you 2 12v plugs.
http://www.toyota-4runner.org/access...installed.html
I've got it in my car - it's really nice on longer trips.
I'm not sure about them bring prewired for heated seats but my guess would be that they are somehow since there were quite a few dealers that installed aftermarket leather and I'm assuming they would want the leather heated. There wouldn't have been so many of them doing it if it required too much work.