The same thing happened to my 81 280SE about 4 years ago.The forward and aft adjustment quit working and the seat was all the way back.I disconnected the teleflex cable from the motor and turned it by hand to get the seat were I needed it.While the cable was disconnected from the motor I noticed that the square shaft end was rounded off a little.I pulled the cable out of it's casing and put the rounded off end in toward the seat.It's been working fine ever since.It's worth a try.If the cable won't turn when you have it disconnected from the motor,you obviously have another problem.Good luck.