It depends on where you are mounting the guage. The dimmer switch is pretty easy as far as accesibility. I tapped into an illumination wire on one of my switches in order to provide light to my 4 new switches. I do not recall the color but I know it was the last one on the plug. It is pretty easy to find. I have a powered tester so I could just send power back through and when I touched the correct one the dash lit up. You could also use a standard test tlight and turn lights on and off until you find the correct one.