OK this is what I recommend
Clay it, Clearkote vanilla or yellow moose by hand, scrub as hard as you want
You can also use a PC or rotary (600-1000rpm max with a rotary) using a soft foam pad and VM
That will strip the oxidisation off, then you have myriads of choices
What I usually do
Autoglym 07B & then Sonus Tire Gel or Klasse SG mixed with sonus or wolfgang tire gel.
Optimum PP and 303 over the top (as many coats as you like)
Forever black (I couldn't get it to do anything on soft plastic)
I don't know what I would do without CK and Autoglym 07B, a brilliant combo
the gummi fledge by 1Z, trim shields or renapur leather care are others I have used on plastics/vinyl and rubber
Over the past year I have restored many a faded bumper and trim with VM and RMG
Watch yellow though, it seems to be the strongest oxidation remover on certain plastics, can take the shiny coating right off if scrubbed on.