This is pretty straightforward and fast thing to do.
**This was done on a BRZ with an auto dimming rear view mirror**
I am NOT responsible for any modification you do to your own car! I am just showing how I modified mine
First off you need to order a product called 'Invisicord' -
http://www.invisicord.com/order.html (Check the installation page to see if your radar detector is supported). I ordered a 12" cable without fuse which I thought would be long enough but it barely was. I would highly recommend ordering at least a 16" cable.
Here is what it looks like (Picture credit - Invisicord)
1 - Start off by removing the dome light fixture
2 - Unplug cord running to dome light
3 - View of what stock wiring to auto dimming mirror looks like
4 - View of stock harness for auto dimming mirror above dome light (I already have the Invisicord wired in photos)
5 - This is the connector you are looking for (front view)
6 - Plugin the prongs from the Invisicord as pictured (RED wire bottom right, GREEN wire bottom center) Look how I am holding the piece so you don't do the inverse of my directions -
7 - Simply plug everything back in and tuck the harness the furthest away from the windshield and on the driver side -
8 - Reconnect the dome light wiring and put it back in position -
9 - DONE!!
If done properly your radar detector should now turn on when the vehicle is started and power down when you turn your car off. Best of luck!