I'm no chemist, but I know from personal experience that acetone will permanently stain some plastics (especially vinyl) and cause it to go "gooey." I was afraid to use MEK on my PVC dinghy, but that's what the adhesive instructions said to do, and it cleaned without doing any apparent damage. So maybe that's the answer. Still, I'd try an inconspicuous spot, first. And of course apply something to protect it, 303 or wax or whatever.
I've had good luck on materials like rub rails with a product called "back to black", sold for tires in the big-box stores, but the effect doesn't last long in direct sunlight. You'd probably need to re-apply monthly or even more often.