So I need to hook up an electric water temp gauge and a tach to switched 12V. What is the best way to do this? Put a spade connector on the end of the wire and jam it on to the end of tong holding the fuse in on the radio or other terminal that is only on when the key is on? Jam a piece of exposed wire under one of the terminals that is only on when the key is on? There doesn't seem to be any great way to do it. Suggestions for a wiring dummy like me?

Should the tach ground wire go all the way to the negative battery post, to the engine block ground or just to the nearest piece of metal under the dash? How about my ground wire from the mounting stud on the back of the water temp gauge? Should that just go to the nearest piece of metal under the dash?

There should be three candelabra connectors under the dashboard, one for 12V, one for accessory, and one for lights. Should be red, yellow and orange respectively. Just use a male plug connector and plug right into the appropriate ones. For ground, you can probably use any ground, I ran a wire to the area right by the parking brake where a number of other grounds are connected. I did the same with the temp ground. They usually say to go to and "engine" ground, but plain old ground worked fine for me.