Normally referring to a 4th gen V8 mustang, but is also taking in to account the douche that is usually driving it. May also be describing a mod-ed later model mustang (turbo charged mustangs may be equipped with a 'BRO'off valve). Or the behavior of a 'normal' person once they begin driving recklessly in a mustang (they become a Bro). Bro's usually talk in the third person. Usually WAY too much car for the owner but they insist on adding more power.
Travis: Dude, I hate riding around in that Brostang!
The Bro: Hey Bro!!
Travis: Oh hey bro. How's the Brostang running?
The Bro: Good Bro, but the check engine light is on.
Travis: Oh its probably just your "BRO"2 sensor.