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

I found an NOS upper radiator hose for my 4 bbl. 1965 GTO.
It was on eBay.

Keep looking, they show up.

interested in the turn switch sent an email
thanks Bryan

David, I read you have a 66 parts car, any chance it has the headliner bows? I'm in need of only one (my car was missing just one) but would be interested in the complete set if you have them.
Let me know, Bob

OK, I understand, Take care

Scott, Your address just showed up on the forum or I missed it with the new so called new format. Will get thenm out Tuesday. You can mail a check to: david sorrow 376 Hill Road Abbeville, SC 29620 or pay pal using my email address: davidgto@hotmail.com

Scott, I never got an address to send them to and figured you bought some elsewhere.

Dave Have not seen those bezels,did you send them yet,let me know Scott

Veracka's Auto 26 Pleasant street Lyme N.H.03768