Should be something coming off that metal panel with the red buttons inside the control panel, as the degauss button I think will be on there, you dont necessarily need it connected as the monitor will work without it, but if the colours are messed up a degauss usually fixes, usually you would get that from rotating the cab after or while the monitor is powered up. From memory those red buttons also do the service/test for what would have been the Naomi board originally and there was a loom that plugged into the end of the board that went to the JVS I/O for the service/test buttons and had a plug hanging off the loom for the degauss cable.
Also in the your pictures in case your not aware, the monitor is upside down, chassis should be at the bottom and cables coming out on the left if looking directly at the screen.