ok got it. I'd missed that extra hole in my rear panel to the left of the number plate, and all my light sockets are correct. The reason I thought I had one wrong socket [earlier post] is when I first got the car, someone had poked that fiber into one of the turn signal lights, but that never added up for me of course - [monitoring one turn lamp??]
The only thing not quite right is my number plate light housing doesn't have a hole in it for the fiber. May have been changed?
Did the housing come with the hole for a fiber, or did the factory drill an extra hole at assembly?
If you 'read between the lines', the manual suggests that the housing hole should already be there, as for the rear sheet metal hole it says " 9/16" Dia. hole - drill at asm."
No mention is made that a hole of a particular size needs to be drilled in the housing.