I would remove the trim panel on the windscreen vertical post and run the cable from under the dash up the post, I would then find a way to get access above the head liner and run the cable to the sun roof control panel and from there to the mirror.
Start by removing the grab handles and the door trim and the sun visor, then see what else is holding everything in place.
Additionally you can use a wire coat hanger to pull the cable through or to "fish" for the cable.
Thats where I would start, I hope this helps you out lol
Julian.