I can tell you that most electrical engineers make the worst phone line repair guys...
Anyways, at night park somewhere dark, sit in the drivers seat and look in the rearview, and get someone to hold a bright flashlight and shine it through the rear window onto the mirror.
With no light hitting the front sensor, and lots of light hitting the sensor facing the rear, it should start to darken slowly to a green colour.
The flashlight should look greenish in the rearview.
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