I installed some glow gauges that needed to be hooked up, though for the life of me I cannot recall the voltage though it should be the same. I spliced into the headlight wire, in my 2000 GT its the brown wire with a white stripe located in the junction box behind the headlight switch. For mine there was no negative wire, the other wire was a ground and I placed it under the dash near the fuse box, I just wraped it around the screw there and tightened it. I reccomend hooking up the gagues to the headlight wire, so whenever you turn on your lights, so will your dash.
Are you using the el-glo gagues? Thats what I have. The instructions they provided really were no help, I had to figure it out myself.
I would make sure the voltage is correct before hooking up. Use a voltmeter before doing anything, dont want to blow your transformer in the gagues.