Back of the headlight sw.
Have you seen this page? -> http://stealth-automotive.com/docume...%20Install.pdf
Here's the thing though, you either just use the same wire that dims all the other dash lights (put a volt meter on the wire and "dial" the brightness up & down on the sw to see if the volts change to know you're on that wire) OR you use another dimmer with
that wire (Isspro sells a kit) to "off-set" the factory dimmer. See, they use different kinds of light and, you might find the gauges too bright compared to the rest of the instruments. You could also experiment with resistors and put a 5cent piece in-line to save on using the Isspro kit.