2] Rear Seat Removal
Each seat on the door side has a pin secured by a thin metal clamp. Remove the clip and be careful not to drop it down into the rear wheel covering. On inside where both seats meet each other, there is a plastic strip. Slide it up and remove the three screws.

1] Vac for seats
I couldn't find anything in the124 Repair Manual, but not to say it's not there. I'm curious how the system does get its vac. I suspect, as you do, that it gets vac from door locks pump [under right rear seat].