Fuse #3
 

My 1972 left low-beam will not come on.

I pulled the fuse box, and I noticed that the rear spade connector did not have a wire going to it. I looked at the wiring diagram, and it does not show a wire going to the #3 connector, but it does show a thin black line between #3 and #4. Is the spade connector supposed to be left empty?

I just replaced all the fuses. If it is not the missing wire, what else should I check?

There is no wire going into the #3 fuse.

What you see in the diagram as a thin black wire between fuse#3 and #4 is representing the buss in the fuse block that attaches these fuses together. If you look carefully at the fuse block where the wire connectors are, you will see that #3 and #4 inputs are actually one piece of brass (a brass strip connects them).

A common headlight fault is the ground connection. It is a 6mm stud (10mm nut) located on the rear of the headlight bucket (inside the trunk area).

Ah yes. I just rechecked the diagram, and #4 is the right low beam. Makes sense to have these fuses paired together. I wonder why there's still a spade connector for the #3 fuse, though.

All other headlights are working, high beams work. It's just the left low beam, so unless it has a seperate ground, I doubt the ground is the problem.

Yes, exactly. Each headlight is grounded on a bolt at the top of the corresponding headlight bucket. The left and right headlights do not share a ground. High beam and low beam grounds through two seperate wires stacked on the grounding point, so one could be good and the other bad..

Ever thought you might need a new bulb? Check for continuity on the filaments.