Seat back is usually secured at bottom by small screws in metal tabs beneath lower corners at each end and one in the middle. The unit is hanging via gravity on "clips" along front edge of hat shelf area. Remove screws; lift unit up and off of hanger "clips". Very easy. Good luck.
BTW, if it has been removed before, the middle screw may be missing. That has been the case with two of mine.
91 350 SDL. 81 300D