As I was driving home from work last night, all the lights on my 1980 GL 1600 went out. Only the hazards/turn signals continued working. The fuses are all fine in the box but the (15A) headlight ones are not receiving any power (tested via electrical pen). However, when I use the pen to arc from a working fuse to those ones, the headlights work. What should I look for next?

Take a look at the fusible links on the strut tower in the little black box. the black one is the main. There should be a red and a green one, also.

 

The headlights ground thru the switch.

I agree with MilesFox. The fusible link connections are a common place to have bad connections that can cause this kind of trouble. While your checking that it may be a good idea to also clean the battery connections since that is another common place for problems that a lot of people neglect to keep clean.

Thank you all for the help it worked out perfectly!

You're welcome for the help. Thanks for the update and glad you got it fixed.