The driver must have been wearing a tank top or wife-beater in order to allow for easy access to the auxiliary hairs.

Now we address the source of the flame. My guess; zippo lighter.

Why? Because it was a Ford Bronco (last made 1996, valued for off-road capabilities) with 5 guys crammed in, one of which we know was wearing a wife-beater. So far we have a profile of a redneck. Rednecks like zippo. Zippo lighters are known for producing a flame by simply lifting the top. Voila! instant flame with which to burn the pit of his dearest friend.

How was the hair lit without the driver noticing before it was too late? Terrible country/rap was blaring through the speakers, the boys were feeling free and invincible. Pre-frontal cortex had shut down after track 5 of Luke Bryan's latest crap album, one kid decided "Hey! I've got a lighter, and his armpit hair is showing. Light + armpit hair = fire. Fire is hilarious and will make the other guys think I'm a funny prankster"

We know the rest of the story.