Buy yourself some cheap mics and foam ruber ear covers from headphones, you will make a heap of footage just going out for a drive and testing.
Foam rubber isnt the best insulator, I nead to make what is called a "windscreen" for obvious reasons, it looks like a shaggy dog that covers your mic.
Drill hole in the side of your gopro to take the 3.5mm mic plug.
Ive tried quite a few diferent places, inside the tow hitch with towbar removed, taped to the back of the tail gate, under the bonnet, in the cab.
you will note the mic cord and the safety cable to a screw in the petrol cap area.