I USED THE OUTER SPACER PLATE, I PUT IT IN PLACE AND MOVED IT DOWN THE OUTSIDE UNTIL IT FOUND A SIMILAR CURVE IN THE BODY OF THE C5, YOU WILL KNOW WHEN IT FEELS MARRIED UP.
Hold that there and mark the two holes.
I drilled the hole a bit smaller than i needed, then made them a little bigger so that when you attach the fracture plate to the inside the two lugs are a tight fit. (dont put to much pressure on the lugs as they break off very easily).
Then put the bolts through, attach the nuts and then slide the mirror down engaging the nuts into the slots. The bolts will fit into the slots and prevent them turning when you tighten the bolt from inside the car.
The official fitting instructions are contained in the "unofficial c5 handbook" which can be found in the download section of the c5alive site
http://www.c5alive.co.uk/downloads2.htmHope this helps.
John