I have installed dash cams (same Anker ROAV as pictured in this forum link below) in my 2012 528i (also in a 2017 Explorer Platinum) by tapping into power in the rearview mirror harness. I'm going to install another one into my 2016 G12 soon, just haven't got around to it. Here's a discussion showing the harness to wiretap, along with a wire tap link (phone/ethernet style connector typical for radar detectors) from amazon:
https://g30.bimmerpost.com/forums/sh....php?t=1440445
Here is a link to show the process to remove the mirror harness cover:
https://www.newtis.info/tisv2/a/en/g...les/1VnYPIzTKZ
And for what it's worth, here is the Dash Cam and wire harness I have used (I realize you're asking about a radar detector, just throwing this out there):
https://www.amazon.com/gp/product/B0...=kinjadeals-20
https://www.amazon.com/gp/product/B0...83RV6W5M&psc=1
The wire tap in my link converts from 12V to 5V (USB power) in case any radar detector is similar.
Just use a volt meter to test your wire harness connection for voltage and polarity before hooking anything up just to be safe.