Factory Tach Wiring

Ok,
I am working on my dash for my 69 383-s. I have always had an aftermarket tach (25 years) placed in front of the factory tach.

I don't even know if the factory tach works. I have located the two wires that connect to it. One is a dark blue with tracer that supplies power from the fuse box. The other is a gray wire with tracer that goes through a grommet in the firewall by itself and should go to the negative side of the coil.

The question I want to know is. What terminal does each wire go to. I took the tach apart and I am not 100% sure the spade end went to the right terminal and I don't want to fry a perfectly good tach if I hook up the wires incorrectly. One lug is brass and one is steel.

Does anyone know for sure which wire goes where?