For Mustang:
http://www.mustangseats.com/faq/faq.htm#8b
8b. Washing, Waxing & Treating Seats and Accessories
Mustang seats and accessories are made of the highest quality vinyl and will retain their looks for years without any extra care.
Mustang seats can be washed with any mild, non-abrasive detergent. Mustang recommends Protect All™, a do-it-all type of cleaner that cleans, polishes, waxes, treats and protects every surface. Protect All’s water repellent Carnauba Wax Formula provides durable protection against all elements. The quality of carnauba wax, incorporated into the formula, is largely responsible for providing durable protection. Simply spray, wipe, and polish dry for a beautiful, durable, lasting shine.
Protect All’s™ anti-static property is ideal for treating vinyl to a clean, dry natural appearance. It leaves NO oily film to attract dust and dirt. With regular use, Protect All’s highly effective UV absorber additive helps to protect all surfaces by absorbing the UV light rays and blocking them from reaching the finish.
http://www.protectall.com/mcs.htm
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For Corbin (AND Mustang when on occasion needed):
The Corbin item AlanH suggests works fine for Mustang vinyl but it is not needed every time for vinyl. When you wash the bike, just wash the vinyl seat with the same mild bike-wash or car-wash soap and rinse.
Supporting data from the Corbin website:
Vinyl cover: Vinyl is an extremely durable and weather resistant product and therefore requires little care. As with leather, we strongly suggest that you avoid any cleaners or conditioners with a petroleum base as they will lead to fading and eventual breakdown of the material. Cleaning can be achieved with Corbin Saddle Cream or similar product.