I did this yesterday to my 300D. I could forego some of the steps listed here because they apply to W123s without height adjustment. I didn't even take the seat out completely, I just undid the bolts at each corner, lifted the seat up just a little bit, and stuffed a pool noodle under there, while scraping some skin off of my hand, which must mean I did "something" right. I'm not sure if I stuffed the pool noodle in there like the photos show, but there definately is some improvement, although it still lists to the left side a little bit. I even found a bit of foam in there from someone else's previous attempt to firm up the seats, I left it in there because it was in the back of the seat box, and I needed it in the front. I'm probably going to go back and reposition the foam tommorow and wrap part of it up with an old T-shirt so rear passengers won't find green foam on the rear floor!
-Joe
P.S. If $1.30-1.40 is too much for you, I saw some pool noodles at Dollar Tree for a dollar, or less. Save yourself 30 or 40¢+ for something else.
