Here's the article that been referred to in other posts. It shows how its author replaced his stock radio with an aftermarket unit and how he connected the stock backup camera to it. For whatever reason, however, his method for backup camera connection didn't work for me in my 2013 LE with my Pioneer AVIC-8000NEX.
http://alexapps.net/toyota-sienna-aftermarket-gps-navigation
http://alexapps.net/toyota-sienna-aftermarket-gps-navigation