If you remove your headlight switch and disconnect the harness, the large grey wire in the center of the connector in the rear of this unit is the switched positive wire. Connect the switched power wire from your gauges to this wire. I found a splice tap to be great for the job. Then on the right you'll see a screw running into a horizontal pillar. Just connect the ground wire to this screw and you'll be set.
I actually just did this yesterday so if you need any help, let me know
EDIT: are you doing the entire gauge cluster or overlays? If you're doing the entire cluster, it's a orange/white wire. There's also one running to your radio, which may be the easiest to find.