12v source

I'd put them on a "Key On" circuit. Technically you could go directly to the battery, but like most things, there are always a few different ways you can go, one way to do it right.

I've always used "Add-A-Circuit" fuse gimmicks to add 12V power. See the pic below. You simply pull an existing fuse and replace it with this gimmick. Notice that it holds 2 fuses - one for the original circuit and one for the new circuit you are adding. The short wire from the gimmick is now your 12V power. Simple!

In the past I've referred to my Haynes Repair Manual for which fuse to piggy back onto - it lists all the car's circuits in the back of the book and will tell you if the circuit is always hot or only when the key is on. I am looking around and cannot, at this moment, find it! I'm sure others will pipe in as this is a common way to "Add-A-Circuit"!

Best of Luck!
Chris

PS - one issue I've encountered is that your Fuse Box lid will no longer fit over the unit due to the tall gimmick, but it's out of site, so this is not much of an issue in the end.

That "add a circuit" looks like a very dangerous device to me. A 20amp fuse requires a certain gauge of wiring to handle the current safely. The wire gauge and fuse are a matched pair, to ensure the wire doesn't overheat or cause other problems. Unless you keep the total amperage between the fuses the same (for example, replacing a 20A fuse with two 10A fuses) then you run a real risk of causing an electrical fire on the wire from the battery to the fuse box. If a fire starts behind your dash, your car is DONE! And don't even tell me most people use it correctly. If the factory fuse says 20A, you aren't going to be able to easily use a 10A fuse on that circuit because the load probably draws nearly that much.

I would highly highly recommend not using that device. It would be FAR safer to tap into a wire after the fuse. At least then, the fuse will blow before your wiring is in danger of causing a fire.

Of course, you have to consider the load you are using. Does the MSD device say what the current draw is? I would think it isn't significant, maybe 1A? If so, choose a circuit that doesn't have huge power draws (avoid the rad fan, blower fan, radio, AC compressor, and rear window defrost, windows) and be sure to tap into the leg AFTER the fuse. It should be dead if you remove the fuse.

For larger loads, you can easily add your own fuse and wiring. I have a nice 1 or 2 gauge wire with a 40A fuse to power my stereo.