I have an adj HD backrest on my 09 Ultra,just replaced the cable mechanism cause broke.Now the backrest will not stay in position when moved back and forward. Any thoughts on why??

 

Just a thought.Does cable have some play so backrest can lock in position?

 

It sounds like the cable isn't moving all the way out. Verify the fixed part of the cable is in the bracket all the way at both ends or it will pull a little.
It's only a couple parts, leave the seat off and watch it, should be relatively easy to figure out.