There is only one ground for motor to start. (battery negative cable to motor bolt, you said you cleaned/checked that). If it won't turnover AND no lights. You have a bad battery or a charging system that is not keeping it charged, or both. After battery charged up, check for it to be 12.6V at rest and about 13.5V-14.5V when at high idle. (if it starts). Also a bad battery can show a charge of 12.6V but will fail after a few starts or drives. A load test will show that type of cell failure.

Get a meter that measures volts DC.

Switch from Delcotron externally regulated alternator to Delco 10si. Throw it away with voltage regulator and all that silly wiring.

Also: The other day I saw something that made me think of you. I saw a picture of a GMC instrument cluster that had been fried. As in: shorted out somehow. All of the PC copper runs were black and chipping off. I think that is what you had. I think the black stuff was the protective plastic sheet getting burnt/melted right above the runs and I thought is was a coating on your other post. Now I think it was burn trace. Charred "plastic".

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn