Mustang. I bought my solo with detachable back seat and removable back rest used for $200 in near mint condition. BEST investment on my bike...

 

Corbin also makes a two-up with backrest for driver and passenger. They are very expensive, but there aren't a whole lot of choices besides the Mustang. Also, Mustang doesn't offer a passenger backrest from what I can tell from their web site.

-Beer

http://www.corbin.com/harley/xl/hdxlcu7gl.shtml