"Auto-darkening" rear view mirror, doesn't

[Half-cocked]

My '99 Mounty's rear-view mirror has the automatic-darkening feature, so that you don't have to manually "flip" the mirror up at night to reduce the glare of headlights behind you.

The problem is, it hasn't worked properly in ages. It only dims when, and only when, there's a vehicle practically right behind me with high beams on.

Is there any way to adjust the sensitivity level, so it darkens quicker? I haven't been able to locate a sensor to determine if it's obscured, which is a possibility, I guess.

 

[reapereviltwin]

The sensor should be on the mirror facing the windshield, look for a small "eye" which faces the windshield. If this is obstructed, it will interfere with the dimming feature. I am going by a self-dimming mirror that was on a silverado, but the basics should be the same.