I've not heard of this before, but I do have something you could check...
There are two light sensors on the inside rearview mirror that control the dimming action fo both the inside and driver's side outside mirrors. One sensor is below the mirror looking out the back window, and another is on the windshield side of the mirror looking out the front. The mirror dims when the sensor looking out the front sees dark, but the sensor looking out the back sees light (glare from headlights behind you.)
If that light sensor lookiing out the windshield is blocked or covered, the mirror would think that it's dark outside, and the daylight that it sees from the rear would be interpreted as glare, thus it will dim.
Look at the back side of the mirror for the light sensor. It is a small hole in a depression a little smaller than a pencil tip eraser. Make sure that it is not covered by a sticker or something hanging from the mirror. Also, make sure that the hole is not filled with dirt or some other obstruction.
My guess is that it's not a fuse... I think that the mirror will go clear if it doesn't have power.
Is the mirror clear or dark when the ignition is off? I think it should always go to clear when off.