I scored one from the salvage yard recently. It attached to the two side screws of the center console, and two previously unused (plugged) bosses in the floor, on the sides of the driveshaft hump. On your (newer) 240, I believe those bosses are where your seatbelts anchor.

It's a nice addition to the car - comfy for relaxed cruising, but you can fold it up out of the way for more 'active' driving.

Reset for service light on an 89 is awkward. You have to remove the felt cover under the drivers dash. The reset lever is on the back of the spedometer, which you can identify because it is white and on the back of the instrument cluster. If I remember right the lever is black and very near the wiring harness for the spedometer. Once you know where it is you can reset it by feel without looking, but you will have trouble if you don't know what you are feeling for. If you want a permanent fix remove the warning bulb. As for the armrest I have never seen one.

1) The armrest is attached using the two bolts that hold in the seat belt receiving pieces (what would you call those???). It goes right above the handbrake and folds up so that you can have access to the coin / cubby hold in the handbrake console

2) There is a little toggle switch behind the instrument cluster (actually right behind the speedometer). Remove the plastic screws holding up the felt at your knees and feel up in there until you can push the switch. It's easy once you've found it.