The bright green wire is high beams and the brown wire is low beams. Sounds like you have is crossed up by they headlights. I don't know, maybe GMC is different than Chevy?
Just remove the bulb behind the indicator and use oncoming traffic as your indicator. "Flash-flash", "Oops, sorry!"
About half the cars on the road nowadays don't seem to have high-beam indicators in them. I think they left them off when they removed the turn signal lever...
