I don't remember the color codes, but the harness feeds regulated DC (5 Volts, IIRC) to each row sensor, and there is a "signal return" wire and a ground wire to each sensor. There's an infrared LED and a phototransistor in each seed drop monitor with a gap between them through which the seed falls, causing a "blip" on the signal return wire as every seed drop through the sensor.
SO... 3 wires...
+ Voltage (PROBABLY red)
Signal return (Green???????)
Ground (PROBABLY black)
Do ANY of the row monitors work? if any do, try swapping the sensors to rows that aren't working and see what happens.
Once you figure out the color scheme you can check for voltage at each sensor. The third signal return wire leads back to it's own row-specific pin in the harness-monitor connector.
Oh, yeah, take a good look into each row sensor for dirt, seed treatment debris, mud dauber or spider mess or ANYTHING blocking the "eye's" view of the LED light source.