When I bought my bike used I threw the backrest in the garage and said no way that's going on this bike (I also said no way the windshield is staying on). Tried it one day about two months ago and it has not been off since. I too have serious back issues and it is a huge improvement.
I got my backrest from Zanotti's for $147.35 + shipping. You might get a better deal from Chicago HD, thier shipping rates are cheaper.
All hardware required comes with the backrest. A very easy-straightfoward install, takes about 10-15 min.
It's just like American Express, guys. Never leave home without it!! Been using a backrest for since 1988. Don't think they were hardly available before then.
The mount y ou buy from HD is really adjustable for those of you who feel crowded. There are several different sets of holes that you can use. If you're tall, I'd suggest setting it as far back as possible and then adjusting the screw in the backrest to let it lean back. You can pick up a lot of room. I'm not tall by any means 5-9 or 10 and at first I set mine all the way to tlhe front and it scrouged the hell out of me. I've since kept adjusting it back until i've got plenty of room now. My back is really screwed from working construction for years and I don't think I could ride very far with out the backrest, I love it. Stafford