I have a 63 Dodge Dart, and want to install a tach. The instructions tell me to hookup the red wire to something that is only hot when the key is turned on. And then the white wire to hookup to the lighting circuit. I dropped the steering column, and pulled out the dash panel, to see the wiring back there. I see a huge red and black cable coming to the back of the panel. That appears to be the power for all the guages in my mind, but I am not sure. Then there are all the wires coming from the engine department, that I recognize as going to the temperature guage device on the front of the engine, etc... Off to the side of it, there is a smaller red cable going to the back of the panel. My thinking is I can use the heavy red and black cable for my power and ground for the tach. As if my assumption of it powering the guages is correct, that turns on only when the key is turned on. Then that smaller red cable would be what powers the lights, and I could tap into it with my white cable, that lights up the tach. Then run my green cable to the coil. Does that sound about right?

I think you really should look at a wiring diagram. You might consider powering the gauge through an accessory circuit.

The after market radio in my truck was installed and powered using the seatbelt light/buzzer circuit. The previous owner had completely disconnected the plug end from the idiot light. This effectively killed th light, the buzzer and made the radio only work while the engine was running. I assume they probed for power with the engine running instead of looking for power with the key in the accessory position. I'm just glad there wasn't enough electronics behind the dash for them to tap into and break anything critical.

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The Old Goat '83 D150 225-2

AKA Ice Titan

I found the wiring diagram. That small red wire was correct for the lights. Now I am working on the radio wire for the red wire. Luckily both are on the ends, so easy to find. But I still don't see on the wiring diagram those bigger red and black wires I spoked about in the last note. But oh well. I shouldn't need them.

The thick red and black wires sound like "Battery" main feed going through the alturnator gauge, both those are 'hot' all the time. (that black wire is not ground)

You are on the right track, go to the radio circuit and splice into that.
BTW: if you want the tach's light to be tied into the dimmer circuit, along with the rest of the dash gauges, look for and tie into an orange colored wire.
DD

IT WORKS!!! Thanks. I finally got it all hooked up with the red or orange (couldn't see it that great) wire for the light. And the radio wire for the tach. I turned on the engine, and is was working. I turned on the radio, and it still works. And I turned on the lights, and the light came on in the tach. I turned down the light, and the tach did too. So, it is done, other than I just ran the wire going to the coil in a way I don't want to keep it. So I will fix that. Thanks for all your help.