Interesting project.
Guessing a little, seems that 15 pin connector expects a proprietary video source.
Pin 4 probably feeds power to the monitor, but you need to know if it is meant for 5V, 12V, 24V...48V... DC...I do not think it is AC 400Hz from the pins description.
Pin 3 seems capable of accepting plain composite video directly when the monitor is placed in that input mode, probably with data fed to
Pin 12 to set the mode, and
Pins 5, 10 to the power supply negative
Pins 1, 2, 3 are to feed VGA signal instead of composite when placed in that mode by probably pin 12 above.
Pins 6, 7, 8 are for the shields of the RGB video signal when used in VGA mode
Pins 13, 14 are also for use in VGA mode.
There may be a default mode that perhaps needs no data input at turn on. If you are lucky, try feeding composite video to pin 3 as is.
Or try to investigate with manufacturer.
A discarded VGA monitor is a good source for the 15 pin cable to start hooking signals. Just make sure with a continuity meter and label each wire-to-pin and carefully observe polarities.
If pin 4 voltage is unknown, try with 5V first, then go to 12V. No more.
Come back with findings,
Miguel