The Story: I am a competitive SSB 64 player, i have been playing since 2004 and i am top player in South American. My problem is, i only play with keyboard. last year i began competing in console using n64 joysticks with... not the best results, so i imported a Raphnet GC to 64 adapter. Thanks to that i improved my performance but it just isnt the same. The Raphnet adapter allows to map the GC Dpad to the N64 stick,s o i though that with a bit of soldering i could build a custom "keyboard" for the GC and then use it in the n64.
This is the result (ignore the cheap pulsators and shoe box lol, ghetto solution but i needed this for tomorrow and in less than 10 bucks haha):


So far, it works perfectly and as intended, but i need a couple of extra buttons and specially, the start button. I already know where and how to solder the L, R and Z button, but i am totally clueless about how to solder the START button. I only see one place to solder the signal cable, but i a clueless about where to solder the ground. Any help is appreciated.