I decided to hardwire my Escort Passport 8500 radar detector into my E90.
What I used was the 8500, and
Escort's Smart cord to be disassembled.
Started off by removing the front lamp module assembly
Disassembled the unit
Took off the "airbag" cover
Drilled a small hole for the mute button
Hot glued the smart cord in place
Melted a hole on the side so that I could run the cables through
Took apart my 8500 and wrapped it in antistatic bag
Hid the radar unit in the roof liner and reassembled everything
Ran a wire from the fuse box to power the smart cord/detector
You will loose the ability to detect Laser but its works great so far, You wouldn't be able to tell I have a radar detector in my car.