backup camera!

[yarismo]

hi all, i got a dvd player on my yaris supporting the backup camera with a wire on it to detect the position of reverse gear.
i installed the camera and all the wiring except that on for camera control, it should be connected to the reverse gear circuit to automaticaly switch to camera mode when the reverse gear is selected.
the question is, where can i find that circuit ! it should be somewhere near the gear shifter, but i cant see where exactly.
thank you for help