How to clean the canvas roof of my Lincoln Towncar?

I've got dark spots on the navy blue canvas carriage top of my Lincoln Towncar that I can't remove. I've used soap, water, white vinegar, carpet cleaner, spot remover...no luck. Some dirt washes off, but the main stain is still there. I park under an oak tree, so perhaps it's giving off some type of goop that's landing on my roof? Any help is appreciated.|||about your only option is ajax and a stiff brush and if it doesn't come out with that you will have to re-dye the top or replace it.