Installing a boost gauge and the instructions say to tap into the dimmer switch. I really think there has to be an easier way than taking the dash trim that far down to access the dimmer switch. I was looking for advice on how others have wired up aftermarket gauges to light them up.
Where did you install it? There are many places to tap into that particular circuit. The switch is located close enough. Otherwise if you plan on getting it from the instrument cluster, its really not that much to get down to it. What year mustang, and where your mounting it will help us out more.
Look at a wiring diagram or just peruse the fuse box and see which wire going into it powers the dash lights and tap it there. That's what the instructions for the Ford Racing gauges told me to do on the one's I put in my wife's old Cobra.
As noted, at the switch itself (be sure to get the dimmed portion of the parking light circuit, as both will illuminate the test light when the parking lights are on) and the leg of the circuit at the back of the radio suffice.
Look for Light Blue/Red wire on the radio. It should be the dimmer for the radio. You can use it to power the lights on your gauges. Be sure to use solder and heat shrink tubing on any wire splices you do. See Ford Fuel Injection » How To Solder Like a Pro for soldering & heat shrink tubing tips.
I use that wire as well, then it will dim like the stock setup, like Hissin stated you can grab it at the headlight switch as well...blue with red tracer