My 66 has been in the shop for over a year. I just got it back. Beautiful paint job but now my left tail light won’t work. He thinks it’s the ground but doesn’t know where or what to ground. It worked when I took it to him. Any suggestions would be greatly appreciated.

 

To test if it is a bad ground, run a wire from the taillight housing to a good ground. If the light works it's a bad ground.

 

There's only one ground for the entire rear of the car. There is a black wire that attaches to the side of the housing of each light socket at the rear, including the license plate lamp socket. It runs up to the front through the rear body connector and grounds to the instrument cluster and then to the Z bar via the radio connector black wire.
If the other lights work, that ground is good. So the only place it can be a bad ground is at the socket of the non-working tail light. (Assuming it is a tail light we're talking about and not a brake light, that's a whole different scenario) If the turn signal and brake light work on that side, you can eliminate the socket ground. If the one tail light is truly the only issue, then the problem must be in the socket or bulb. Either the nub on the bulb isn't making contact to the socket nub, or it is corroded, OR the filament in the bulb that operates the tail light is broken. Assuming you have changed the bulb, (hopefully), you need to check out and clean up the contacts in the socket. Everything else can be eliminated just by verifying what works and what doesn't. IF it's only one tail light, the problem WILL be directly in that socket, or the brown wire that goes to one of the connections in the socket for that lamp is broken or corroded.
All you need is 12v on the brown wire and ground on the bulb base for the bulb to work, so that's the area you need to work in.