On the inside fuse box there's a pretty good size wire that goes to the rear window defrost. I tapped off that wire and so to run my mirror heaters, I just hit the rear window defrost button and it turns on mirror heaters and rear window defrost. If your truck has that option, you could do that if you don't want to add another switch. I've had mine that way with GMT800 mirrors and now have it with the new mirrors also. I went to a junk yard and bought the pigtails(8 wire instead of our 4 wire) off a heated mirror Suburban. The newest style mirrors are heated on the top and bottom mirror. My GMT800 mirrors are only heated on the top mirror.