To get to the number plate take out one of the number plate lights and file a slot in one corner near the back and put back in to see if there is enough room for the thin wire to go through(not RCA size) If you cant see through the hole then file a bit more out.
Poke a piece of wire through with the end turned over and pinched. So there is no sharp edges then poke it through the rubber and tape the rca to it. Put lots of tape to taper the plug down to wire size. Then lube up and wiggle it through. I think I came out the side of tube at the end and then went around the rubber seal
There is a small gap under the rubber boot seal between the outside body and the trim
Ill have a look if your still stuck.
Putting the camera above the number plate is still the best spot. I couldnt think of anywhere else on the back to go.