Mach 1 Club

Trying to get an idea if I have a headlight or wiring problem. On high beam all four lights work & on low beam, only the driver's side, outside light. I thought low beam lights were single element fixed brightness so one would think it would not work on high either? Thanks
(1969 Mach 1)

The two outboard lights have two filaments, one for low beam and one for high beam. So it could be the bulb.

Probably is the bulb. You could move the driver's side over to the passenger side first to see if it works before buying a new one.