Typically there are 3 contact points on a tachometer... power, ground and sense. The power is the hot lead (+12 volts) the ground is the ground and the sense lead goes to the negative side of the coil.
On your tachometer (which is set to 8 cylinders), here is what I see. The two top terminals are isolated from the case by the plastic stand-offs. This implies they are not the ground lead. That leaves the bottom terminal as the ground. Under the top right terminal is the word “IGN”, which means ignition. That appears to imply the +12 volt (power) lead. The one remaining is the sense terminal which goes to the negative side of the coil. This is my belief. Others may have a different point of view.
It would be easier to analyze if the wires weren’t connected and we could see what was behind the red wire.
In all honesty, that red wire implies a hot terminal but it is not connected to the “IGN” side, which gives me some pause.
