On my Tempest, the outer part of the lamp socket was the grounding source- it was poor, so I scuffed the area on the grounding ring and on the housing- BINGO!
IDK if the GTO has the same setup, but I'm gonna guess it does.
If you can't get it to work, take the lamp socket out of the housing (I'm assuming the socket just untwists vs. permanently attached) and put the bulb in, then attach a wire from a good ground to the grounding ring of the socket or the grounded part of the bulb- If it lights, you have a bad ground.
If it doesn't, you'll need to verify that you have +12v on the supply wire- just use a simple test light, or a multimeter, either will work.
Let me know how it works out-
John

Saw your plasma tail light post. I put mine in , but they will not work in my 65 GTO. What do I need to do??