You actually are in greater advantage than you know. 780s with the original tool kit have a funny little crank terminating what looks like a square male drive speedometer cable, If you pull the kick panel carper fasteners at the bottom of the front seat back, the carpet will come up and expose what look like female radio antenna connectors. By now you have guessed that if you plug the drive crank into the connector and turn, some part of the seat will move.
Most problems are the switches in the side pocket console which are just other versions of the window motor switches. Cleaning them uses the same procedure and results in a fix.
There is one motor in the seatback for tilting it, the three motors under the seat, from front to back, are rear section tilt, seat forward and back, and front section tilt. The forward back motor has two attached position sensors that could be the culprit, yet more likely it is the switches or a wire cut by the rack for which there was a recall.

Bonjour,
I'm not in front of my 780 right now, but I did have a similar issue when I first picked up my 88 780. My driver's seat would decide to move to the full front position while I was driving (not very safe).

I pulled over, and when the seat started moving forward, I pushed back with my legs and moved the seat position button to the rear, and the seat started to go back. Now it then allowed me to move all the way back to the full back position.

Now I had to quickly hit the big red STOP button to eliminate the electric motors from trying to move back to the front.

Hopefully that will allow you to move the seat back. I'll look to see if my service documents have any clearer pictures.

Let us know.