The slightly thicker diameter green wire under the dash is constant. It runs in one of the groupings of wires right under the steering wheel. Removing the panel will help a ton.
tap the wire on the dimmer with variable output for the lights ... you have a voltmeter or test light right? I don't remember the color I just check for the right wire with a test light ...
use the yellow or green wire in the ignition harness to power the guages. the yellow wire is hot in acc and run where as the green wire it only hot while the key is in the run possition ....
There is also a purple wire for the ignition, that only comes on when the key is in the "on" position. Used it for my seat heater, and that would probably draw far more current than gauges would.
At the risk of sounding ridiculous, what are you guys using for grounding? I'm relocating my gauges to the center console and I don't want to tap off of the radio's ground for fear of whine.