It seems like the procedure would be to test if you have power at one point, perhaps the combo switch like what Siteunseen suggested. If you have power going into the switch, see if you also have power going out. If you do, then work your way to the next connection and see if power makes it that far.
I've got a '74 so the electrical system should be similar. There is a 3 blade connector on the combo switch with a red/white wire that I think should be hot when the headlights are on. Check to see if it is. If not, and power goes to the combo switch, the switch is the problem. If it has power, work your way to the C4 (Brown) connector and see if there is power going in and if power is going out the other side. Keep looking for where power should be and if it's not there, you've isolated the fault.
It's a long shot, but try cleaning your fusible links.
When you are done and found the issue, look into adding a headlight relay.
Good luck.
Jeff