I use a homemade mount out of PCV that attaches to the rear crossmember;
How To Make An Awesome GoPro HD In-Car Mount For Approx. $30 (AK370Z)
For audio I use a seperate audio recorder and splice the sound in using Power Director. I have the recorder inside the car so there is very little wind noise and the audio quality is better then what the Go Pro can do.
Putting the camera on the front bumper is a good location too, gets it nice and low