Ajax or Comet, but don't leave it unattended ....was off quickly after scrubbing...I use to use it on the bottom or rear from exhaust splatter on my white 61
As stated above..."don't leave it unattended". I would add to the list Bleche-Wite and a good stiff brush. Be sure to keep all painted surfaces wet at all times and rinsed often. And what ever you do....do not use / or complete this job in bright / hot sunlight. Its better to do this on a cooler / overcast day or in the shade, just to ensure the painted surfaces do not get too hot. Try it , you will be amazed at the difference.
Raggtopp is environmentally safe, spray on application, scrub lightly with a brush and let stand before rinse. Raggtopp is safe for paint, though I'm not sure if the same can be said of Comet - not to mention the gritty, abrasive powder that is difficult to rinse 100%.
If the top material is vinyl (rather than canvas/fabric), you should give it a good coating of Nu-Vinyl or Mequiar's polymer protectant for plastics, once you've got it clean and dry. That coating will keep loose dirt from imbedding in the surface, prevent UV damage, and make it clean up without scrubbing in the future. Just re-coat it about once a month after washing the car.