It could be the camera, the trigger or the head unit itself.
To test the trigger:
Pull the head unit out and find the small harness PZQ60-00150 it should be labelled, if not it is the one that has just three wires and one is green and this is the reverse signal, tap into that with a pin and a multimeter. with car running put it in and out of reverse a million times watching the multimeter and the screen. you should get 12v and camera on. If you get 12v and no camera on, problem is the camera or the head unit, if your not getting 12v you wont get camera on and you need to follow that wire and fix the connection wherever it is tapped into reverse signal.
Even with a good trigger, the camera needs to be ok. After a van ran into the back of me I cut the camera wire to remove the camera, trigger still connected to head unit but it was as though no camera, the map would still display.
Hope this helps.
Typical Toyota not to look into the problem, if not their fault yes they could quote to fix it of coarse but no they just give it back instead. My car came out from a service with loose leaf spring u bolts, i found them the day after, rung toyota and asked WTF it says on my receipt suspension bolts checked(or something to that effect) but the reply was "if they are aftermarket we wont look at them" hmmm thanks, you will surely be getting my business post warranty period, not.
You could always just throw it all away and buy an original one from ebay:
http://www.ebay.com.au/itm/NEW-TOYOTA-G ... bf5&_uhb=1