For the next guy that comes along (cabs and coupes only):
The rear seat had to be removed (bottom and seatback). The seatback has two screws inside the armrest cavity. Then remove the side panel by unscrewing one attaching screw at the bottom of the panel and lift the panel out. You have to tug it pretty hard to get it to detach at the window seal. It will still have the seatbelts running through it, but you can do what needs to be done with it set aside. You can see the assembly that works the extender. In my case, all I had to do was to push the extender arm back into position so that the gear mechanism could get its teeth into the arm. There is a limit switch at the top of the assembly that can easily be checked while activating the mechansim by opening and closing the door. Check the nylon cog that extends the arm. I found a link where some company in Colorado will manufacture a new gear if theat is needed:
This has some pretty good instructions and photos of the removal process for a 560SEC.