Could be fuse, the mirror itself, key unlock warning switch, or wiring harness itself. That's what it says in the repair manual.
The manual says to pry open the top cover of the mirror with screwdriver (it's a small rectangle panel on the roof of the interior, it's not the mirror but it's the thing where the wire goes from the mirror to the panel). The panel should then have 4 terminals (square slots). It says to put positive (+) lead from battery (you're car battery, or any other 12V battery) to terminal 1 (that's the left most terminal. The negative (-) goes to the middle left terminal.
Then you "Press the mirror switch and check if the mirrors surface repeatedly becomes 'bright' and 'dark'. If operation is not as specified, replace the liquid crystal inner mirror".