There are 2 manufacturers making backrests for the OEM seat:
- BAKUP - http://www.bakupusa.com/YAMROADSTRAT.htm
- Utopia - http://www.utpr.com/roadliner.html
- GrassHopper - http://www.grasshopperlimited.com/
- Fire House - Firehouse Backrest Images
Here is a link to pix of a forum member's Fire House install with a non-Backrest Mustang Solo Seat
A search for Firehouse Customs Backrests doesn't turn up a current source, but can often be found on Ebay
One of our members may be producing a clone of it.
Note - These backrests may require some modification (drilling holes, etc).
The other option is to purchase a new seat with the corresponding backrest. The following manufacturers have seats which fit the 'liner AND support backrests: