Right at the outlet of the fan assembly. You can possibly get to it from the inside by taking off the cover of the fan. You may have to take the fan out of the housing to do so. If it isn't too hard, I would suggest you go into the attic and look at the outlet of the fan where it comes through the ceiling.
Be careful where you walk/crawl! Keep your weight on the ceiling joists and don't step on the drywall, or yuo'll have a much bigger job on your hands. Or worse, hurt yourself.