For 1990-2009 cars

When the ignition switch is in the ON position, B+ is applied to terminal 11 of the inside rear view mirror. When the transmission is placed in the REVERSE position, B+ is applied to terminal 10 of the inside rear view mirror. Ground is applied at all times to terminal 8 of the inside rear view mirror.

The electro-chromatic portion of the inside rear view mirror uses two photocell sensors to determine the light conditions in the front and back of the vehicle. The mirror automatically darkens to reduce the glare from the headlamps that are behind you. A photocell on the back side of the mirror senses when it is dark outside. Another photocell, which is built into the mirror surface, senses the glare from the headlamps that are behind you. At night, as the glare of the headlamps behind you increases, the mirror gradually darkens to absorb the glare. The mirror returns to its clear daytime state when the glare from behind is reduced.

The electro-chromatic portion of the inside rear view mirror has a time delay feature. This prevents rapid changing of the mirror from a darkened state to a clear state as you drive under traffic lights or through heavy traffic.