The best (IMO) way of doing this is to use an accessory's voltage as a signal for a simple 12Volt relay (slightly higher amperage than the expected draw of the Halo/Demon Eyes). An accessory is anything that gets power going to it when the ignition is on (make sure it's not getting power all the time too). The most accessible place is the climate control panel, specifically the clock or AC pot. You can check which wire you want to tap into by using a multimeter with one lead in the back of the connector and one lead on a known good ground. Also you may way to add in a switch between the signal and the relay just in case you want to turn them off at any time.
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