I hope it turns out to be as simple as replacing the switch assembly with a new one, but I'll be a bit surprised by that. It seems like the buttons that see the most use would fail well before the less popular buttons. For none of them to work would typically hint to me that it's likely a wiring or connection issue.
I remember taking mine apart many years ago to rewire it to work with my aftermarket JVC car car stereo. Unfortunately that was back before decent digital cameras were affordable for a new college graduate so photo documenting my projects didn't happen much and I don't remember if the buttons were tact switches or metal domes/discs. If they were tact switches, rebuilding he assembly would be pretty easy and affordable. If the metal domes, it can be done but may require a bit more labor to rebuild circuit traces.
-Rod