When I first got my car I talked with a upholstery guy who fixes these tops. He told me the best thing for it was Ivory dishwashing soap. I tried it for a while, and while it kept the top clean, it didn't keep it black (started to fade).
Now I use a high pressure washer on it to get the all the dirt out, and use
STP Sun of a Gun Protectant to get it good and black. I've been doing this for about 4 months now with no problems... even won 1st place at the local Mustang show. Hope I could help.
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'95 GT Convertable