On my unit there is an earth for the camera circuit which should be connected to a reverse circuit earth so when reverse is selected I think it provides an earth for the camera video display power on the unit. It is wired this way so you can't watch it when driving as that may not be legal?(ie reverse only) So in theory any earth will do, if just want to connect it to test. At the camera end there is the video lead which connects to the head unit, a supply lead for the camera which needs a 12v supply (reverse light cct ?) and an earth for the camera supply.
That is how mine works I think, currently in the middle of fitting it, it was a sod to fit the camera in the number plate light slot on the barn door so still needs connecting up.