I'm using this external mic
http://www.amazon.com/gp/product/B00...=5336432695-20
Works like a charm. I route it from the chasecam headrest to the floor, then through the back seat, spool the extra cable in the trunk and tape the mic on the bumper facing downwards.
The standard GoPro2 plastic enclosure has no hole for the external mic so you have 2 options. Either buy their "skeleton" enclosure which is open on both sides, or drill a hole on the plastic enclosure and be done with it. I went for the latter, I can take a pic tonight if you're interested.