Welcome to the forum Jeff !
Last time I searched for carpet in plum / purple I found some close match's in "Pink" .. kind of a dark pink looks close to plum.
I've been told that auto carpet is same as marine carpet as far as UV and water proofing.
From the other thread you posted about Burl wood ..
Avery Dennison use to make some that came close to original, but is out of stock at vendors or they quit making it.
Last search I did for "Burl Wood" came up with some lighter stuff.
Also if you want to wrap your seat bases, you'll need some wide stuff because of angle of seat base fronts.