What would it take for the warranty to be invalidated?
I am thinking of having the reversing camera retrofitted. Would this be reason enough?

It would have to be proved that the fitting of the camera caused the problem that you are claiming for. So if your gearbox fell apart it couldn't be blamed on the camera fitting, but if you had a short in the electrics it would still have to be proved the the fitting of the camera was to blame. p****d off with a Digital Keyboard Warrior