On interior lights ....at least on our cars, the power wire is constant. The lights are switched on by the switches giving them ground. I just used an extra interior light assembly like this.
I just screwed the light housing to the brace under the rear deck. Ran the power wire to it. Then ran the ground wire to the switch. It's just a simple universal door jamb switch. When the trunk opens, it gives the light ground and turns it on.
