Sometimes, you need to try different methods. I had an old Schwinn that was covered in yellow paint. I soaked a rag in carb cleaner and applied it to the frame. The yellow paint wrinkled up and I was able to wipe it away. It wouldn't affect the original black paint, but if I left the rag on too long, it would affect the factory pin stripe.
I had another one that a friend used his thumbnail to flick off all the applied paint to get to the original stuff. Probably not something you want to do to a full size truck.
