From the service manual
INSIDE REARVIEW MIRROR WITH THE AUTOMATIC DAY-NIGHT FEATURE SYSTEM OPERATION
The inside rearview mirror uses 2 photocell sensors. One sensor is the headlight sensor, located on the rear side of the mirror. The other sensor is the ambient light sensor, located on the front of the mirror or windshield side. With the automatic day-night feature enabled, the mirror uses the ambient light sensor to determine the exterior light condition. With a low light condition detected, and a high light condition from behind at the headlight sensor, the inside rearview mirror will automatically darken the face of the mirror. In the daytime, the mirror is in a normal state because of the high light condition that is indicated by the ambient light sensor. With the gear selector lever in the REVERSE position, backup lamp supply voltage is supplied as an input to the inside rearview mirror. In night time conditions only, the mirror monitors this input to disable the automatic day-night feature which allows the face to gradually change to a normal state. This allows the driver to clearly see objects in the mirror when backing up. The mirror also disables the automatic day-night feature when either door is opened and the courtesy lights are illuminated.
I hope this helps
See you Saturday,

arty Looking forward to seeing your UV.