Need to find 12v power source...

right at the switch, u can use the constant power going in and the switched power off the parking light circuit, your gunna need a test light or a scematic to know which wires they are.
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pull the fuse out you want to use and just push it back in with the wire underneath it so it makes contact inside the fuse box. you will know when the gauge either works or doesn't if you got a good contact. I stripped my wires back about an inch before pushing the fuse over them and back into its spot.

you can get fuse taps at Autozone if your using a larger size fuse. I bought these to use but the fuses in my panel were all the smaller size ones and the taps wouldn't work. don't know if they make ones specific to the smaller fuses. if they do this makes for a nice cleaner install into the fuse box.
 
I just installed some indiglo faces in my car.

I grounded them on the metal bar behind the cluster. I beleive there is a screw to the right you can unscrew, loop the wire around and tighten up for a solid ground.

For a dash light on power source, i used the brown wire off the headlight switch i beleive. It came on with the parking lights.

I used a crimp from radio shack that allows you to splice onto the wire without cutting it.
 
its not the most beautiful or cleanest way to do it but it works. the fuse taps would be much cleaner but like I stated above am not sure they make them for the smaller fuses like our fuse boxes use.

you could just get some of the piggyback connectors and crimp them onto your switched source at the headlight switch but remember to stagger your piggybacks since you will probably use the illumination from the switch as well.