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I always use auto-dimming compass mirrors. I get them from junkyards fairly cheap - I've put one in my truck, my brother-in-law's Bronco, my girlfriend's MR-2, and I have a few more in case I need them. They're TOO cool! Advance & online places want ~$250 for one!
You can tell an auto-dimmer by the lack of a dimming toggle at the bottom of the mirror, an electric eye front & rear, and a small wire conduit running down the glass to feed power. Most rear eyes are in the plastic, but some look thru a hole in the mirror, usually top center. Some have a disable button sticking out the bottom.
You can recognize a compass mirror by the same conduit, the magnetic sensor somewhere near the glass, the large (~quarter-sized) rectangular hole in the R upper corner of the mirror, and the select switch at the bottom. Most Fords use a double pushbutton, and most GMs use a 3-position slide, but you'll find both types on all makes.
Some mirrors only have ONE of those functions, but most compass mirrors are auto-dimming. A VERY few also have temperature, but I've never found one in a junkyard. Most require their own size button to mount to the glass. All only need ground & switched 12V power to work, but some have other features that use more wires.
They can be found in Crown Vics, Grand Marquis, Town Cars, older Explorers, some Buicks & Olds, and a VERY few Caddys.