Some plastics will work with the shoe polish & coffee method, but some do not. My DiMarzio pickup covers wouldn't work using any method I tried, so I taped them to a window for a few weeks in front of a windowshade to allow additional light to reflect onto it. In a month the difference was what I had hoped for.
I don't recommend leaving them in your car's rear window. I tried this and they melted big time after just 1 day. :P
"It's funny the way most people love the dead. Once you are dead, you are made for life." - Jimi Hendrix