How to run electrical wires in an exterior wall with a low-pitched roof?
I've run into this often and have become very successful at getting into corners of the attic I never thought I'd be able to. Sometimes you have to flat crawl on the joists.
What has also worked is cutting out a square in the wall inside the room at the ceiling height and using that as a pull station. You will probably have to drill through a sill plate but then shove the wire up into  the attic, lots of it. Then go into the attic with your snake or a 10' piece of EMT with a 90 degree bend at the end and snag the wire and pull it toward you. Feed more from the inside  room or pull out the slack. Use the same hole in the wall to drop the wire down to the switch location. When finished, repair the drywall using the piece you cut out. At times, you might have to cut the square into the ceiling too.