if you’ve had the problem from the initial install take it back, usually with aftermarket stereos you have to feed a wire from the stereo into something in the car that tells the stereo your in reverse (usually the live wire for the reverse lights) maybe be been hooked up to the wrong wire, if its flickering I would probably take a guess the link between the car and stereo to indicate this is incorrect and probably loose or not attached securely if its flickering.