I got a '79 240D in pristine condition from the original owner, a89 year old lady. The driver's seat had a broken spring, the other front seat looked like it was never sat on. I pulled out both seats, and found that you need to change the backrest, as the center armrest is fastened to the back of the driver's seat. I removed the seatcover from the damaged seat, and put a piece of 1/2 inch plywood between the broken ends of the spring, and added 3/4 inch foam rubber under the upholstery. it made the seat more comfortable, as the original horsehair was shredded under the seat.
olweasel (retired marine engineer)