I couldn't face trawling through all the old posts, but here are the pictures of my 13 pin swivel mounting. Rear bumper removed for clarity (and I've only just collected my car from the body shop

)
I have to say that despite there being a couple of holes for you to use to mount it, I couldn't find any suitable so that it could be hidden without fouling the lower edge of the bumper, so I welded it to the towbar (that's what the new black paint is on)
up out of site (or it would be if the bumper was fitted

)

pulled down as in use.

I might even paint the gubbins behind the plastic bumper ....... if it ever stops raining