There are two other things I would check before worrying about the button controls. One common failure item is the resistor pack, wich looks like a bunch of coiled wires in a cube (I don't know where it is on your car) and the other is the "strip fuse" which is often in its own obscure holder (sometimes near the shoch tower unerhood) and is held in place by two screws (visible once the holder's top is flipped open).
The cicuits do fail, but their failure is not that common. As for more troubleshooding and disassembly, maybe Rivermasternc can chime in?
\'87 Mer-B 300D (220,000 miles) (sold)
\'93 Mer-B 300D (173,000 miles) (sold)
\'87 Porsche 928 (101,000 miles)
\'98 Dodge Ram quad-cab (250,000 miles)
\'2001 CLK430 Cabrio (50,000 miles)
Past cars: 1969 mercedes 250, 1981 300D, 1972 220D