I wired up some gauges in my A-Pillar. The illumination lead is connected to the dimmer (though it doesn't "dim" so I may have the wrong wire), the constant 12V is connected to the cig lighter and the ground is connected to the metal on the a-pillar itself.

The lights have 7 colors, and it remembers what color you were using with the 12V constant. If you disconnect the 12V C, it goes to green. The other night I honked my horn and it went green.

Why?

answer is yes! same fuse controls both.

Really? My cig lighter's fuse is blown, and my horn still works... unless that's for the older cars.