Maintaining Matte Black Finish
A quick detailer (I prefer Mothers) will clean the surface. But if you have a good looking surface, just simply wash with water and a little car wash soap. To protect the finish, I use Mothers Protectant or Protect, forget which but there is only one. If the finish needs restoring, use Back to Black. This is a wonderful product that will restore that factory look as well as remove any white wax residue.
Basically, you want a product that will protect the plastic, but not change the nature of the finish. Mothers Prot. does this as well as Meguiars has a product that does this. But starting with the Quick Detail or washing would always be the first step.
Hope this helps !!!
Tom - Lss Vegas