Back in 1968 i can't remember what I used on my '66 Mustang Vert, but on my 73 grand Prix I used (sparingly - and cautiously) Westley's Bleach White. It worked on the Prix without any damage to the top for 12 years till I sold the car. it sat outside most of those years and was still white at last sight.
Go to the nearest boat dealer and get canvas boat cover cleaner (not sure of the brand name). Did this for my daughters '89 ragtop based on a tip from a co-worker and the top came out looking good as new. It's usually a few drops of this stuff (pink in color) in a bucket of water, scrub with a brush and rinse.
Use Soft Scrub bathroom cleaner. Get the one with the bleach in it. That's what I use on my boat vinyl and it should work just as good on your top. It has gotten some pretty nasty stains out of my WHITE upholstery.
You guys are making me cringe!! Soft scub and steel wool, Pleeeassse stop.
Go to the boat dealer and or Summit and Get the following
"Marine Spray Nine multi purpose cleaner" $7.99 per quart . www.spraynine.com It's PH alkaline for vinyl, and it really works great and not a lot of elbow grease necessary to use it.
After cleaning you must seal the vinyl to protect it from UV degradation. "303 Aerospace Protectant" $6.99 per 8 oz. is the product of choice. It works on ALL vinyl (DASH PADS, Vinyl tops, Upholstery) it blocks ultra violet rays from degrading vinyl. www.303products.com
303 also makes a cleaner and both products are available from summit.
I purchased both products (spray 9 and 303) from the Boat Tree Dealership, in their parts dept.
I was truly amazed as was my mechanic/mustang restoration specialist. at how well the top looked after I used it the 1st time almost 1 year after he installed the vert top. It had a NEW appearance. I chose these products after much research on the subject both here and elsewhere.
303 is the way to go. I have used it since first trying it two years ago, it worked so good that when you spray it on you can watch the dirt run off. With a small amount of elbow grease and a scrub brush the top will look almost new. My top was put on in 1980 and still looked good up until the time I changed it this spring. It was dried out and cracking from the years of neglect but 303 brought back the white color.