REVISED March 30,2018
I recently simultaneously hardwired both a BlackVue 750s dashcam and a Uniden R3 radar detector in my new 2018 540i. The installation turned out to be easy by using a standard size fuse tap to activate unused fuse socket F59 in the Z44 fuse panel located behind the glove box, and required no special tools or modification of any existing wiring. I used a Blendmount mirror mount to hold the dash cam behind the rearview mirror and a sun visor clip for the radar detector.
I have prepared a step by step web page describing and illustrating my method, see
http://www.eyephysics.com/BMWG30/bmwg30.html. I have revised my original instruction since this original posting with pictures of routing the power wires behind the airbag extension in the right pillar. The only tool needed was a T20 screwdriver and a bit of electrical tape to bundle the power wires with the existing wire bundle in the pillar located just behind the airbag. No rewiring or any changes to the original install were required other than simply removing the pillar cover and shifting the wires so as to remain behind the airbag instead of drooping over it.