W123-W126 interchangeability
 

Will the rear seat from a 81 126 fit into my 83 123?
any mods needed?

All you have to do is cut the car in half and add a 3 inch piece of metal -:D
Jack I am fairly certain the 126 bench is longer BUT the 126 bench is 58 inches if you want to run a tape on your 123

Someone on here did it. It doesnt bolt in and doors are hard to close. He was able to do the front.

Will the covering at least fit in the back over my existing springs
or would it look tacky?
I know the front will work, I have done that many times, but I would like
to change the color of the seats. I have found a nice 126 with black
leather and would like to see it in the 123 instead of the Palimino.
What about switches for the power seats? The 126 is power and I would
like to keep it that way in the 123.......is that possible, again without
looking tacky

It won't work without some effort. I picked one up for my '81 300D by mistake and didn't realize until I was putting it in. It is a bit wide, I was able to get the seat back attached witht the screws, but this cause the back to sit too high to hook at the top. The seat bottom squeezed in, but the latches are different so it won't latch. Also, because it is squeezed in there kinda tight, it causes the seatbelt to bind and they don't retract easily. I only spent about an hour fiddling with it, so with a little more work you might get it to work.

I've researched this a bit myself. I have a line on a fairly decent set of leather pinkamino seats from a W126. Looks like keeping power seats in the front could be challenging. The back seat covers could be modified to fit the original seat frames fairly well, but I'm still hung up on what to do about the folding armrest in the back... They hinge at opposite ends.