I have this book on auto electrical systems, a really easy to read book, perfect for newbie beginners like me who didn't know anything about how electrical circuits worked.....the title is "Automotive Electrical Handbook" by Jim Horner, and I got it at Borders for about $20. On page 44, it had a diagram of a typical tach hookup, along with a few paragraphs about it. It said there are basically three wires. One is red, one is black, and one is green. The red one can connect to some place that provides power in your fuse box, the black one goes to ground, and the green one goes to the signal. The signal in this case is a place on the ignition coil that's labeled NEG. or DIST. or TACH. Luckily, the backlight came on mine already when I turned the lights on, so I thought that it just wasn't getting signal. I found the green wire in the back of my tach, along with a black wire, a red wire, and a white wire (the white wire goes to lighting but I'm not sure yet). Those were hooked up, I just needed to find where the green wire went.
I followed the green wire coming out of the tach through the firewall. It was shoved into a body of wires and took me quite a while to find the end of it. I found the end of it finally, and sure enough, it wasn't hooked up to anything. I got this kind of wire connector:
and crimped it down onto the wire, and connected it to my ignition coil. Now, the service manual I had for the car talked about a normal distributor, the stock one that came with those models. But mine had been upgraded to an HEI distributor so I was having a hell of a time finding my ignition coil. I asked a few guys from a chat room, and they told me the ignition coil was inside the distributor. There was a section in the Haynes book I have that confirmed this. So once I knew that, I just connected the wire with its connector to where it said TACH DIST. on my distributor.
I started the car and drove it around for a while, and sure enough, the tachometer needle went up as I accelerated. Success!!