I hate it when you can clearly see damage to something that’s been caused by opening it up to repair or modify it :-( When I bought my first SNK Neo Geo AES console I had to repair the console, only to find the joystick was faulty too! In my case it was just one button not registering (it could be all buttons though).
After ruling out the console, and the buttons and wiring in the stick itself, it was time to tackle the DB15 connector. This is quite easy to pull apart with the righ technique. Cut up a thin piece of plastic sheet, something from a piece of packaging, usually those really difficult to cut open packs that your latest joypad for the PS3/360 came in (you get the idea!)
Looking at the casing it’s easy to see the clips on both sides holding it together, start by putting a piece of the thin plastic sheet in as far as it will go up to the clips.
Insert a THIN flat screwdriver or similar object in-between the plug and the pastic, twist the screwdriver slightly to pull the casing apart and wiggle the plastic strip until it goes up and over the inner clips.
Repeat this on the other side of the plug and you can simply pull the casing apart, done carefully there won’t be a mark on it :-)
The photo above is after I repaired it, the purple wire and it’s connector had popped out of its socket, it was the wire for the Start Button. If it was the Ground wire that had popped out, all of the buttons would have stopped working.