Ben and all,
my car has the jump seat. To install it, both of the rear parcel shelf pieces come out. The seat bottom goes behind the passenger seat and clips onto a metal bar near the middle of the car, and the seat back has a mounting screw with a rubber "nut" that goes through a small hole in the side panel. The nut is installed from inside the soft top boot. If I'm not mistaken (and I could easily be), cars that were not ordered with the jump seat don't have that hole.
The parts of the seat are indeed somewhat complicated in shape, as Duro said. In addition, the upper passenger side seat belt bracket interferes with the seat back, and I can't mount the seat back without removing the upper seat belt bracket.
All of which leads me to suggest this, if originality is not a priority: just make a simple seat frame from two pieces of plywood attached at right angles, and add cushion and trim to make it look nice. Just sit it on the passenger side parcel shelf. Take out only the driver's side parcel shelf to make foot room over there. Wouldn't be quite like the original, but a whole lot easier to make, and avoids the seat belt problem.
On the driver's side, under the parcel shelf piece, is another small carpeted piece of wood that makes the floor where the rear passenger's feet go. That would be pretty easy to make.
Good luck, whichever way you go!
George Davis
'69 280 SL Euro manual