Pull fuses one at a time until you have 0 volts at coil + with light switch on, ignition off. That won't test all circuits, but it will eliminate a lot of possibilities. If none make a difference you'll still need to continue to underdash connectors, but fuses are an easy place to start. Also pull anything tapped into the IGN or BAT tap on the fuse box, if it is connected to another source it will back feed.