How To Wire a Tachometer?

[savatreatabvr]

I was searching Craigslist the other day and came across a factory tachometer & fuel gauge combo unit out of a 1975 1 ton truck. I thought it was a good deal for $20 because it came with the factory wiring harness so I bought it. My problem is am I missing part of the wiring harness and do trucks produced without the factory tach/fuel gauge still wired for a factory tach/fuel gauge? Meaning is the wiring harness the same for all trucks with or without the factory tach/fuel gauge? I'm putting this in a 1973 Chevy C20 454 and can't figure out where the 3 individual wires go that are pictured below?