Look for something like a radio power, or anything that is ACC power (power when keyed on, but cuts power when off). To test it, you need to have key off, check that there is no power, then key on, check that there is power, then key off and check that the power is once again non-existent. I believe for my gauges, I tied directly into one of the sources powering the radio, since I ran all my wires from behind the radio, when the radio came on (key on, or up to ~10 minutes after key off - if the door didnt open) the gauge stayed worked. Once the radio was off, so was the gauge. Worked well for me.
