I have drilled two holes in the back of my 1968 CA160 fender for the dual filament bullet light turn signal and running light. I used a fender trim from a 1965 CA95 fender trim as a fixture to drill the holes. The holes were then enlarged to fit the light stud. At this point I may attach the light to the fender or use the CA95 trim as a light base. It is convenient to do this task while the bike is apart. It can be done to a fully assembled bike. So if this is the case you must remove the rear tire. Yes you can run the wires through the trim if taking apart your bike is an issue.
Here are some starting pics.