This is the dash of my El Camino with the instrument cluster removed. The green-black & red wire hooks to the back of the speedometer with a screw that is almost impossible to get a socket on it bacause there is so little room. On the right is the backside of the instrument cluster and you can see the wires I had to add to have gauges. Even though my car is a "SS" it came with just a speedo and a fuel gauge and idiot lights. I bought a instrument cluster from a junkyard but the circuit board was NFG so I just wired the Tach, temp gauge, volt meter and oil pressure gauge to directly.
The small curve on the left side of the metal plate is where the speedo cable has to run through. When I get my speedo back to re-install I don't have to rip it down this far. I put the instruments in about 18 or 20 years ago. The speedometer conversion was done about 15 years ago. I have it all written down in my little yellow notebook. The notebook is on the last page. I will have to go to book two real soon.