I added padding to the lower back of my seat in my 300D. The inside is accessable by sliding the back cover down after removing a screw at the bottom. Check out this site; www.bergsmamotorwerks.com.
These folks list all kinds of guides for the doityourselfer including a Seat restoration Manual.

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'96 C220 138,000mi, '95 E300D 239,000 mi., '87 300TD 214,000mi '88 6.2 Turbo Diesel Chevy Conversion Van 253,000 mi.