Personally, I wouldn't want to hook it up that way... dimming mirrors only dim when they sense bright light entering the passenger compartment from the rear to save your night vision.
If you hook it up the way you describe, it will be dim whenever the headlights are on... therefore, your rearward vision in that mirror will be significantly reduced when its dark out... reducing your awareness of what's around you.
When you try to use your mirror when you back up, you won't be able to see...
Just my 2 cents.

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KenP

1996 S500 W140 Coupe