how do a clean a convertible top on a 1991 mustang?

Soap and water or try one of these ----- https://www.amazon.com/s/ref=a9_sc_1? rh=i%3Aaps%2Ck%3Aconvertible+top+cleaner&keywords=convertib le+top+cleaner&ie=UTF8&qid=1477278746

Is the top cloth or vinyl? Either way I would start with a wash using car wash soap and a brush which will remove dirt a lot better than a cloth or a sponge. After that it all depends on what type of top and what color it is.

I had a white vinyl top on my white 90 fox vert before I replaced it with an upgraded black canvas one & the only stuff that worked was called "bleach-white" it was cheap & came in a spray bottle - I would mist it on & scrub with a scrub brush with a long handle & then rinse & rinse - I lived in total fear of birds that ate purple berries & swore never again would I have a white top! :)