The full current of the headlights pass through that switch and it fails. There is a recent thread, amongst many, that discusses converting it to a relay driven system as the switches are getting harder to find and, well, it is just a good idea.

Also a long shot. The '80 has the 4-headlights (can't remember if they are rect or round) so I would check your grounds for the headlights. My fuzzy memory tells be it has a different grounding point for the high beams and low beams.

Lastly...check fuses...and don't just say they are OK. Take the appropriate ones out of their holders and check their condition and the condition of the holders. Again, fuzzy memories tell me my '79 had power fro two different fuses for low and high beam but I think I am mistaken.