Internal Wiring Diagram ?

Hello, can you link us to which specific items you have so we can look into this further with you?

Here are the manuals / guides we have for this:

and https://www.instructables.com/Retro-Arcade-Cabinet-by-Micro-Center/

They likely created their own wiring diagram but may have used this pinout.

http://www.xin-mo.com/dual_player.html

I spent some more time looking into this. It was simple enough to just connect a switch between the non connected white wires and a ground coming off of the xin-mo controller board since all the wires are already there, where there's a few extra black ground wires which are the grounds as well. After connecting those additional wires to switches, they show up with their input assignment when configuring the inputs through retroarch.

However, here's a problem I ran across. I'm mostly into the 80's arcade games where most of them run best using the lr-mame2003 emulator which is the same emulator the MC Atari arcade games are using. What you will find is that the unconnected wires are J1(Joystick1) inputs (8-15) and J2 (8-15). What I found is although the inputs work once you connect a switch across them, it seems to matter which emulator you're using to launch a game rom in regards to those additional available wires on the Xin-mo controller working or not. For example, I wanted to add a button which launced the config menu screen to my MC Atari console, the same one which comes up when you press the TAB button on a keyboard as well as a "Pause" game button. What I discovered is that the lr-mame2003 emulator will only recognize inputs J1 (1-7) and J2 (1-7) inputs, which are all the same ones the MC/Atari consoles already have connected to buttons on my dual fight stick with trackball. Inputs 8-15 are not recognized/supported by the lr-mame2003 emulator.

I've been having better luck configuring the Atari arcade game roms to use the Adv_Mame and Adv_Mame1.4 emulators instead. When using those ADV-MAME emulators they seem to support the additional 8-15 inputs on the X-mo controller and can then be configured to do whatever you would like. The problem there is that not all of the Atari game roms will run under those ADV-MAME emulators. For example, I found the missile command Atari rom will not run using the Adv-mame emulators.

Earlier it was mentioned of being able to add additional buttons to the side for arcade pinball games which is another thing I wanted to do as well and have done. I expect I will have to use the Adv-mame emulators to make use of those additionally wired buttons, but then hope that particular pinball game rom works with the adv-mame emulator. If it's another lr-mame2003 emulator only game rom, then you're out of luck on that front unless someone knows some additional configurations which can be performed to get the lr-mame2003 emulator to recognize the Xin-Mo inputs J1/J2 8-15?

Overall, I've been having better success launching most of the Atari Arcade game roms with the Adv-mame emulators instead of the MC configured lr-mame2003 emulator. My game screens are not cut off on my HDTV, my trackball, and spinners all work better, as well as supporting the additional Xi-Mo inputs. So far just found missile command doesn't work when using that approach and have yet to be able to get my track ball to work properly while trying to play Missle command while using the lr-mame2003 emulator. Sure wish MC/Atari would have had that configured for me to work properly.