You at least have to drill holes for cables, even if you had a number plate mounting one. I'd cut a bigger hole, and go to the effort and fit an OEM. Available from the Forum shop.
The OEM head unit also seems to do some video processsing on the OEM camera. The OEM camera has a fish eye lens, and gives a fish eye output. The OEM head unit flattens this. Not sure how a 3rd party camera with a regular lens would look on the OEM display.