300SDL soft seats
 

Does anyone have a solution for the soft mushy front seats in the 300sdl? Is there a aftermarket seat that will work? Thanks, bob

I wrote a guide on repairing w123 seats (I think they are basically identical to yours). This is good if your seats have any broken springs, also I added foam inserts from a 124 or 126 - not sure which exactly. This wont bring the seats up to the firmness of modern car seats, but makes them much much more comfortable than broken down ones. I am sure other seats will fit, but will almost certainly require modifications to the seats and or floor pan.
Total cost was well under 100$ for both front seats, though my passenger seat was in fairly good shape.
http://www.peachparts.com/shopforum/detailing-interior/373355-seat-repair-alternative-technique-w123-similar-seats.html

Does your seat have broken springs, or are they just saggy?

My W123 did not have broken springs, but the front corners of the seats were extremely saggy and almost going all the way to the floor. I felt like I was sliding forward out of the seat during driving, and the passenger seat was even worse and listing towards the door at the front right corner.

I cut two pool noodles to the same height as the two vertical coil springs that support each front corner of the seat, then shaved the pool noodles to fit tightly inside the spring. Did this with both seats.

Perfect support now! Nice firm seat, but of course I was lucky that I had no broken springs to begin with and so I didn't have to stuff the seat full of noodles.:)

There's actually a correct part to use, if you don't want to use pool noodles to space the springs up. I think I mentioned that in my guide, but it's for either a 124 or 126, and is made out of dense foam coated in rubber. It slips into the coil springs and does the same job, but presumably wont degrade for quite some time.

:DCorrect way to repair(above) -Pool noodles are still good for only 1 summer Replacing the pad also helps quite a bit.