It seems strange that the "mirror" has died. It works similar to photochromic glasses with a little electics thrown in. Check the fuses etc. The following is from the LR Workshop Manual which may help you determine if it really isn't working;
Automatic dimming
The mirror’s automatic dimming function operates when the ignition switch is turned to the ’II’ position.
A light sensor is contained at the upper centre of the mirror which detects the intensity of light from the headlights of following vehicles in dark or low light conditions. When the light intensity is sufficiently high, the mirror automatically adjusts the brightness of the reflected light by photochromatically changing the mirror’s reflective properties and sopreventing glare affecting the driver.
When reverse gear is selected, the power supply for the mirror’s photochromatic function is opened so that the mirror defaults to its normal reflective properties while the reversing operation is in progress.Another light sensor is located on the rear of the mirror so that the unit can detect the difference between strong ambient light and that attributable to following vehicle headlights.
To check operation of the light dimming function of the mirror, use the following procedure:
1 Turn ignition switch to position ’II’, and ensure reverse gear is not selected.
2 Cover the sensor on the back of the mirror.
3 Shine a bright light on the front (reflective side) of the mirror; the mirror should darken.