...are you talking about the rider's back-rest? Dig your hand in the slot its in, squeeze both sides and pull up. Mine is an '03, but don't hink that has changed, haven't heard of a "fixed" backrest on an Ultra.
Unless they've changed something for '06, no removal needed.
Just open the tour pak, and take a phillips screwdriver and take out the one screw at the back of the seat. Lift and pull back, being careful not to scratch the tour pak w/ the bracket.
Dayum Mega! After reading that question 3+ times, I decided he wasn't talking about the tour box! It did cross my mind, but I've never heard it address that way...."stock fixed backrest". I guess, passenger "stock fixed backrest" would have been clearer.
Between both answers, hope it works out fer 'im......