I fitted a set of 4 to my 57 plateabout 3 weeks ago.
Bought from ebay and fitted in about an hour. Never fitted them before.
As said, 4 holes in the bumper. Splice into the reverse light cable and stick the sounder in back. Cable was to short to reach the front but its load enough.
A little tip. Angle the sensors up a little. Mine picks up the curb or driveways .