Installed a new radio head unit. The factory tailgate camera is working by manual selecting camera on unit. But the reverse is not triggering the camera. After further research i need to tap into a reverse trigger wire at the bcm but unable to locate the correct wire. Thanks 

Simple way to figure this out. Go to the taillights and pull the driver side tail light out and see what colors the backup light is. This color should travel back to the BCM for the control of the light.

I just went through this on my wife's truck.  What year is yours? Are you not using a canbus interface?

2015 ram 2500. Yes on canbus interface. I have seen where people have said if you tap into the reverse wire of the taillights that the interface sends false signal that bulb is out. They have tapped into the reverse wire of the 7-way plug. But these are all situations where someone is looking for a reverse trigger wire at the back of truck not inside the cab.