if I remember right, it is the grey/blue wire coming out of the box thing (I may be wrong, but Ill look it up, and if I am wrong, Ill edit)... this will turn on with your head lights and dim with them when you rotate the knob. You can also tap into this ckt at the fuse box, which is a little easier to do, the fuse is the 15 amp one that is furthest towards the rear of the vehicle, and is one of 3 mounted side to side (rather then front to back like most of them are). This option is easier to do as you can use one of those little tap power from the fuse things that you can get.
If you cant figure out which is gnd and power on the guage, you can always 'test' it running short wires by hand from the battery, or any other 12 volt source (assuming your talking about the lights... other wires I would not do this with, but it wont hurt a light to go backwards).
btw I know mustang wiring pretty well after my escapade trying to fix mine, (really, really long story that will make you want to choke the previous owner of my car) so if you have q's feel free to ask me.
Last edited by Blown94FiveOhAZ; 07-14-2002 at 08:04 PM.