Ummm....been a while since I had mine apart, but I'm thinking I remember that the back rest brackets mount to the frame under that piece of chrome trim on the rear fender while the bolts that hold the bags on are the same bols that holds the fender, bolted through the bag, trim and frame then into the inner fender support....but I could be wrong. Hell I'm frequently wrong...wife tells me all the time. Either way
@Idaho Red Rocket 3 is correct...the factory backrest systems works as designed and comes with new trim that is cut out for the sissy bar bracket.