I also have am external mic(Rode, I think - with a dead cat) mounted on my rear bumper near the exhaust. I run an extended mic cable thru my trunk and back seats to my GoPro mounted on the rear 3/4 glass. It works quite well. I bought the external mic & dead cat as a kit off Amazon - for around $120.
I just bought another GoPro suction cup mount and use that to mount my external mic on the bumper. I use a zip tie to mount the mic's flash hotshoe mount to my GoPro mount. No big deal.
I also have a friend who just puts his external mic in his trunk... that works pretty well too. When you put an external mic under the hood or even a GoPro near the front bumper... you just pick up all the mechanical while of the engine. I don't care much for that type of sound(s).
Here are the results...
My 135i has a Performance Exhaust(PE) which sounds awesome!
Enjoy,
Dackel