This is a petty easy one to do. I finally found an F54 in a junkyard (via ebay!) that had a 40-20-40 split rear seat. The option at order time is S465 "Through-loading system". I'd scour ebay a few times a year to look for someone selling rear seats, then plug the VIN into mdecoder to make sure that it had the right option (as well as base leatherette seats, since that's what mine are).

Retrofit is /fairly/ easy. First, pull up on the seat bottom. There are three clips that must be released, you will hear each pop out when you pull upwards:

Then pull the whole seat cushion forward. It is held in at the back by two tabs (they aren't clipped, they just hold the seat down):



The only gotcha is the center seatbelt: the belt has a fixed attachment underneath the existing seatback. Removing the seatbelt from the seatback itself looks to be a difficult affair, one which involves disassembly the seat portion itself. So, I was quite glad that the unit I had ebay'd had not locked up the seatbelt and it could be used as-is...

Now is a great time to vacuum out under the seat, as well as to retrieve any loose change that fell down into one of the seat buckle holes.

Remove the driver side (in US) cover over the remaining '60' seatbelt by tugging firmly at the top of the trim. You want to pull it forward, toward the driver seat.


The piece will then lift out and expose another torx bolt that holes the '60' side of the seat in place. Unbolt it, move the seatback forward, and slide it out the door.



Install the new '40-20' seatback into place and bolt with the Torx bolt. Bolt the seatbelt bracket into place by first feeding it through the seat bottom cushion. Push the seat bottom into place, and shove down to latch it into place. I found that buckling all the seatbelt up helped in repositioning the seat bottom, since it is otherwise quite difficult to get the buckle receivers to feed through the cushion holes properly.

Et voilla. 40-20-40 seats in a car that didn't previously have them. Now your rear passengers can have a cupholder, or you can load your skis/paddles/other long objects in the back in style.

Total cost of the rear seats for me was $500 (it came as a complete set, which means I now have an entire 40-60 seat back and seat bottom to get rid off...). Worth it to me, maybe insanely over-expensive to others...

 

Nice. Was just looking to do this to our F54 Cubman, with the blue leather interior. Pricing it out is about $2200! I may try and find a used one as you have, and worst case just buy the blue seat covers needed to match my interior, and still be under half the cost of buying each component new.
Part of the struggle of owning a car no longer in production sadly. At least for now it's time to shop for the must have accessories and little things while they're still available.
Thanks for posting!

 

There is a complete set set in blue leather on ebay presently. $1500 for both the front and rear seats (and rear is 40/20/40). You might be able to offer them something for just the rear...

If you're in the USA, I've heard that Katzkin seat covers are great quality and not super expensive.

 

Those blue, purple, and lounge leather options are very nice.