Dash PANEL HELP 1966 Cuda.

I think you'll find there is a same nut underneath the copper shield/heatsink. It's sort of snapped in. pinch it and pull it off. Later models had a plastic cap that looked like a household wire nut on this one stud.
If this is a 4 speed car and there was a reverse lamp mounted in the dash, that's where your wire goes. Only other possibility, Typically on the 2 wires, one is power in, other is power out out of switch to those lamps. Where the power in came from in your year model, I'm not sure.
Most have power in wire connected to a fuse marked as that. In earlier models, it could have been tapped into the wiper switch wiring utilizing that fuse. They did do that sort of thing.
A wiring diagram would help. Should be one available somewhere. Hop this helps.