Just because it doesn't have a reverse camera doesn't discount the possibility of Vauxhall installing generic wiring looms to different models and various options. Which means you could have a wires or plugs sitting behind the dash not connected to anything.
Theres a fakna to din connector in the box that came with the new head unit and because it's got a black connector, I connected it to the other black connector that's the same as the green one in the picture. I will change it over and see what happens.
One of them must be the aerial, usually its the green one, not sure what year your car is but the other must be reverse camera, satnav or onstar, could you connect up your old unit temporary and see which one stops the radio working when you disconnect it?