Is the 5.99 reading with the solenoid energized? A bad connection will show good voltage until a load is put on it. If that checks good, I would install the solenoid and run the car up to engagement speed, verify that the governor is activating the relay, and test drive the car. With the car running at 30+ mph (try second gear) the generator should be charging to 7-7.5 v. The increased voltage and the movement of the gears in the car might engage the solenoid. If that doesn't work, I would run a 12 gauge wire from the battery hot terminal through a 30 amp toggle switch straight to the solenoid and test drive it. If that works, there is a weak connection in the original circuit. If it doesn't work, I'm out of ideas.