Hello.....
I have a 1974 Roadrunner. Four headlights (of course). When the low beams are on, only the right low beam is lit. When the high beams are kicked on, the other three lights come on, and the right low beam goes out. Any suggestions on where to start? Ground? Where? Thanks for any suggestions, Dan

 

I don't think you can plug them in wrong. CHeck the headlights make sure the low beams are both on the right side. After that I would check the dimmer switch/ headlight switch connections to start

 

Thanks, I will start there.....Dan

 

how many connectors are on the back of the low beam bulb. could be that the high beam filament burned out on one side and the low on the other.

 

Mikerp.....
The low beam has two prongs, the high beam three. So far I have cleaned the ground on the headlight harness....I am going to check the switches per TVLynn next......

 

Other way around. Low beam has three

The high beams (2 prong) go in the inner buckets. The low beams actually have two filaments, low and high,

So in low beam, only the low beam filaments in the outer buckets are lit

When you change from low to high, the low filaments go OUT, and all 4 high filaments in all four lamps turn ON

This sounds like a poor ground on the failing lamp.