Oxi-Clean is amazing stuff. Before:
After 20 minutes with a spray bottle of Oxi-Clean and a stiff bristled brush, along with some paper towels to wipe up the really dirty areas (mostly where the back rest meets the butt cushion):
The driver side was the most dingy with the sooty discoloration, which pretty much all came out. I might have to retreat the little drip stain on the passenger side as it looks like it mostly came out, but still has a little bit of a shadow visible. I’ll see how it looks after it’s dry, which shouldn’t take long because it’s 95F and sunny out.