Here is an attempt at detailing how to tap into your day/night mirror power source to install a dashcam or radar detector.
First, pull off the housing cover for the electronics associated with the interior mirror. Put a thumb underneath a corner and pull down. There are 4 snaps that hold it up.
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Do the same on the other side and it will pop loose.
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Once it drops, pull it free and the wiring is exposed.
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Now, unplug the mirror wiring connector. There is a catch towards the back right side. Press the catch and pull the connector out toward the windshield. This will give you easier access to install your wire taps.
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Using tap snaps (the ones I use were a bit on the large size, but they work) and tap into the black (negative) and white (positive) connecting your 12volt to 5volt transformer. Slide the mirror connector back into place (make sure it snaps back in).
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You can tuck the transformer in underneath the edge of the headliner. Snap the cover back into place and plug your dashcam/radar detector in.
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The power is supplied to the device when the ignition is on.
Works like a charm.
I did the same a few years ago to my wife's Edge and my 5 series BMW.
Enjoy!