Another fly in the ointment is the brake/tail lamp with the lift off seat.
The side hinged seats are what I would call a full seat. The newer Specials with the lift off seat have the brake/ tail lamp attached to the grab rail, aka the flying brick tail light, as the earlier side hinged seats have the brake/tail lamp attached to the rear fender.
I prefer the look of the early side hinged full seats so I converted my '83 over and moved the brake light to the fender. Having done this the stock back rest or sissy bar goes on, no problem with the mounts as Skull has stated.
There may be some backrests that were designed to be used with the lift off seat having the 'flying brick' tail lamp.......not sure there......
Now the question to you is which way is your seat attached.....hinged or lift off?
Anything can be made to work with some inventiveness.