OK, first of all, there should be a fader function, so if there’s not one showing up, then there’s an issue. There is a possibility that the car was shipped with the incorrect software. Apparently, the B7-generation A6 shares all of its electronics with the contemporary A4 and the TT, and in the TT there is no fader function. It seems that some A6 models may have been either sold or updated with the incorrect software. I’d suggest getting in touch with your nearest Audi dealer and they will probably have a solution for you.