We are restoring a 1972 Cutlass. Turn on the lights and the two low beams work. The switch has a tan, blue and green wire. The blue has 12 V. When the foot switch is depressed to go to high beam all four lights go off. The foot switch switches the blue 12 V wire from the tan on low beam to the green on high beam. I get 12 V on the green wire at the back of the foot switch. Any suggestions are appreciated.

 

The headlights are grounded to the header panel. Make sure the ground wires are in good shape and have clean and strong contact with the panel. Also, I know this may sound dumb, check that the bulbs are good and make sure there is no corrosion in the plugs.
40 year old wiring can be brutal sometimes.