The Mercedes top cleaner is made by Ragg Topp, but at the dealership it's more than twice the price you'll find it on AG.
303 Aerospace fabric cleaner is what I use, along with a natural bristle scrub brush (boar's hair?) I got here. I wash, rinse and dry the car as normal, then wet the top and spray the 303 generously, spread quickly without scrubbing, let dwell a few minutes then scrub. The foam is visibly dirty, lifting dirt out of the weave. Rinse thoroughly, blot dry and run a fan overnight to dry completely before taping off and spraying at least two coats of 303 fabric convertible top protectant.