Good to see you were able to work out what that button was for :)
On the Japanese import Estima the button is typically located near the driver's right knee rather than on the steering wheel, next to the powerslide door and side mirror controls. I've attached a pic.
The front and rear cameras work well with the factory screen and just operate automatically at low speeds in forward and reverse, so it is a shame yours had had the head unit swapped as it's a very nice feature. I had a 2002 Caldina GT-4 with the same system, very handy for parking and at intersections with limited visibility.
