Edit: I've changed this as it's not as good as I originally
thought it was. The instructions from ProjeKt D are
much better so I've changed this accordingly.
Here are the ProjeKt D instructions courtesy of VirginiaJim
Start by prying off the front cover. Start at the front lower side and once it "pops", DO
NOT pry up any more. Try to pull the cover straight forward (as in the normal direction of travel of the bike). Continuing to pry up will break the upper rear tab tips.
Once the cover is off, you’ll want to remove the three Torx bolts on the bottom.
It is very easy (was for me anyway) to drop the mirror so have a good grip when you remove the last screw.
Once the mirror is off put the Torx bolt back in the hole opposite
of the locating pin, and put a screw driver long ways between them and rotate the swivel either way.
(sorry, no picture)
This will raise it up somewhat from the mirror and give you more room to drill or file
a hole or notch for the wire.Route the wire over, under, or around the mirror and into the inside and then down
through the tube and through the hole.
Then run it between the plastic cover and
the metal mirror mount. Being careful not to squish the wire (tuck as you go) run the wire
down throught the hole and fish it through the bottom of the mirror mount plastic cover, and remount the
mirror to the bike using the three Torx bolts. Leave enough slack inside the mirror so
there is no binding if the mirror is moved forward or rearward. Complete the hookup
to the signal wires and then test the signals and install the painted cover.