Wanted to say thanks to hmmwv1 for this thread, and TNriverjet for the tach (got it from you off another site.) Got mine wired up this past weekend and it works great at idle. Didn't get to take it up to speed due to another issue. I wired mine the same as hmmwv1 to the tach drive, but ran the light power to a toggle switch on the radio mount. The tach power and ground hook up underneath my radio mount which already has 24v running to it from a relay connected to the ignition and batteries. I'm going to run the loose wires seen inside a plastic shroud piece, just wanted to make sure it works before doing that yet.
The mount isn't pretty, but works ok for now. I ended up using a chunk out of an old 3 hole front bumper I had. The 3 holes in that bumper are the perfect size for the tach, so it saved from having to buy a large hole saw for it.