Hi all-
I realize this is 2 years later, but I am new here and was just doing some searches that related to my car, etc.
I cleaned by VERY soiled white top on my 90 GT with everything and still couldn't get the black marks off. I used Boraxo brand hand cleanser (its less harsh than regular cleanser) and water with a soft brush to clean the top overall and that made it very white.
I still had some black spots from where the top goes up and down. I found Goof Off (in the paint section of Walmart and/or Home Depot). I put a little on a toothbrush, applied, then immediately wiped off with a wet towel. Once all the spots were gone I washed the entire top and applied protectant. As spots reappear I now just put a little Goof Off on a rag, wipe the spot off and then wipe and/or rinse. So far this has done a very good job of allowing me to keep the top clean and looking very good.
I hope this helps