I fixed a friends M12S a few weeks ago with the same issue. He tried contacting both Sanwa and Serpent (US Distributor) with no luck.

Fixing it was not overly difficult and can be fixed in about 15 minutes. Using a meter, check that you get a closed circuit when you press the switch down. In my case, the switch remained opened indicating the switch was bad.

Remove the four screws that mushroomed showed. There are two additional screws that hold the circuit board to that cover piece, take those off also.

Micro switch that was not working.



I unsoldered the switch from the board, i found however this was NOT required. Using a your fingernail or a small screwdriver, you can carefully pry back the two plastic tabs that hold this small metal switch cover off.



You can see the contacts of the switch were very dirty, so it was not making a good connection. No pictures of this part, but i used a brass wire brush to clean the contacts of the switch housing and the spring / bridge contact. I then reassembled the switch. The plastic tabs holding the cover piece were slightly deformed from disassembly, so i used a VERY small dot of CA glue to make sure the cover does not come off.



Soldered back on (Again, you can do the above without removing the micro switch from the board).