Simple Green works great. I spray it on my tires and rubber trim every time I wash, it works great to clean rubber goods. Mix it 1/1, spray on the trim, & let it sit for a few minuites. Scrub it with a wet rag and rinse it off, you can see the black on the rag after you scrub it. I put a coat of Turtle "ICE" wax on the rubber trim when I wax my truck.

 

I used water and a rag. A little more elbow grease needed but it does the job.