Wiring Issue Headlamp not working Continued...

If you examine your (darn poor) circuit diagram, notice the top lamp in the drawing, wire L9-18BK. This means wire no. L9, it is no. 18 gauge, and BlacK. Notice the eyelet connector pictured next to the lamp. That is the ground.

L3-18R going to Y of the connector and
L4-18BK going to Z of the connector are the hi and the low power feeds to the lamps, coming from the dimmer switch.

If you get into the service manual, and look up the instrument panel wiring, find the high beam lamp, you will see that it is fed with wire L5, and if you follow that back to the bulkhead connector at Y , you will find L5 and L3 on the same terminal. This means that L3 is the HIGH beam power going to the lamps

The only one left is Z and L4 so that has to be lo beams power