Hi,
Not sure if you have solved this already, but have some info for you. I successfully fitted a single speed motor to my 1980 moke temporarily. One of the terminals on the motor goes to a black twin-wire ground coming through the firewall near the motor (though any ground will do). The other terminal connects to either the Blue Green or Red Green wire coming from the switch. Either works and does not change the fixed speed of the motor.
Cheers, Graham