Rear seat removal...
...I've not done it on 124, but I'd wager it's similar to 128 sedan.
Which I'll try to describe from memory...
1. Remove lower cushion first by lifting very bottom/front edge.
It has couple large locating pins which point down
and engage to recepticles in floorboard.
Might help to also push backwards at bottom/front edge.
2. Remove upper cushion by bending body tabs that engage to
steel rod/framework of the upper cushion...these are below upholstery.
Then lift upper cushion off the body hooks that engage higher on cushion backside.
About a inch is all you lift.
3. Rear package shelf is secured to body by about 4 small body tabs,
located about an inch below horizontal surface of the shelf piece.
Bend those straight, and the shelf should slide forward & out.
It's a snug fit below rear glass and roof pillars.
Caution...if same as 128, it's made of flimsy/fiberous stuff,
with texture pressed into it, and then thin coat of color.
It's also contoured, so cutting a flat panel to fit would be a challenge.
I'd sprayglue a sheet of vinyl over the top.
Hope that helps!