I can only get on via my iPhone so forgive me if I missed something. I didn't see where you posted a year or model of truck. (this iPhone screen is small. Maybe I missed it)
I have the wiring and fuse info for my 04 2500 HD and I looked up the mirror circuit for it.
There is no single fuse for the mirrors on it. But if you check in the fuse panel in the side of the dash inside drivers door you should look for fuse #3 DDM fuse. That's the driver door module. Also circuit breaker 28 lower left portion of the fuse panel is a 25 amp breaker for the whole drivers door. There is also a RT DOOR CIRCUIT BREAKER 25amp listed at "I/P Relay Block neat left kick panel" where ever that is.
I have a picture of the mirror wiring diagram for the 04 silverado if you want it email me.
Hope it helps