Sometimes, upgrading mirrors can be as simple as plugging one with more features in. For instance, most GM vehicles have either a 7 or 16 pin harness and typically have similar wiring layouts.
Other times rewiring and soldering will be required.

If the mirror isn't plug and play, you'll likely have to splice wiring and harnesses to get a new mirror to function correctly. Different vehicles have different colors for the same functioning wires. Without accurate wiring diagrams, it is impossible to successfully install (trial and error is NOT recommended) one of these mirrors without damaging it. I can send an image with the approximate pin out of the harness if requested after purchase to ease installation.  You will still need to find a wiring diagram for your car to complete the process.

If your vehicle came with one installed, it's possible the mirror harness is prewired for it already. The default location for temperature sensors in nearly all vehicles is in front of the radiator, between the radiator and grille area. If your vehicle doesn't have one, you'll have to run the two wires from your new mirror to the installation spot for the sensor you buy. GM vehicles with Onstar equipped mirrors typically use sensor part 15035786, which can be found here on Ebay. Or, I can include a tested one with your purchase for an additional $20. Older GM and Ford vehicles use a variety of styles of temperature sensor.

The feature reduces eye fatigue, especially during interstate driving. Those who have mirrors equipped with this feature say they can't imagine wanting to drive without it. Below is an example of just how much a mirror will adjust to remove harsh lights from the driver's vision. Most of the time, the mirror will never get this dark unless you have a train or something else extremely bright behind you.