Trying to add a 15-pin Neogeo/PC joystick connector to my Sanwa modded DC arcade stick, as some people in this old thread seem to have done, but I'm having trouble with adding the ground switch needed to keep the DC part from interfering. I assumed "cutting the ground" meant cutting the common ground lines to the stick and buttons, but even though it stops working with the DC when you flip the switch, the Neogeo controls are still messed up (everything seems to stay active at once no matter what you press). I assume I've misunderstood something or possibly misidentified the ground lines. 
Could someone please try to clear this up for me -- which line(s) do I need to add a switch to?
Could someone please try to clear this up for me -- which line(s) do I need to add a switch to?