Here’s how the headlight mechanisms work (this is off the top of my head, so there might be some omissions or screw-ups):

The headlight motors have two breakers on the crank that rotate with the motor. These two breakers switch whether current from the up/down control wires get to ground. These control wires have a diode each, which ensure that current from the up wire doesn’t go to the down and vice-versa.

When these control wires hit ground due to the position of the headlamp motor crank, it turns on the headlight motor relay which sends voltage to the headlight motor. The headlight motor spins until the breaker comes in contact with those up/down terminals. The circuit is broken and the motor stops.

Now, the following connections are from memory, so I could be wrong. But, terminal 30/51 is the lead which goes to the motor and is charged by the fuse box (2/B on the ’74). 86 and 85 are the up/down leads (doesn’t really matter which). 87b is ground. 87 is the power lead which charges the relay.