Some gauges require constant and ignition power, but an spst switch will work. Some people hate them but the "add-a-fuse" products work well for simple electrical work. If you dont want to crack open the fsm and get to splicing and heat shrinking then hit up a parts store and find the "add-a-fuse" stuff. That will give you a power wire from the fuse box you can run to your switch and then run a wire from the switch to the gauges and of course run a solid ground wire.
There are several options but something like this will work:
Cooper Bussmann/ATM fuse tap (BP-HHH-RP) | | AutoZone.com
Edit: If you just want the gauges to come on at night you could tap into the illumination circuit from the dash or possibly the radio, but use a fuse or you risk damaging the radio if something shorts out with the fuses. Its more work but you wouldn't have an extra switch sticking out anywhere.