While it may be funny to many to flash your bum at a friend while in a car, doing so can get you charged with a Class B misdemeanor.  Flashing in public is treated the same as if the defendant was having sex in public or publicly masturbated.

U.C.A. § 76-9-702(1): Lewdness

(1) A person is guilty of lewdness if the person … performs any of the following acts in a public place or under circumstances which the person should know will likely cause affront or alarm to, on, or in the presence of another who is 14 years of age or older: (a) an act of sexual intercourse or sodomy; (b) exposes his or her genitals, the female breast below the top of the areola, the buttocks, the anus, or the pubic area; (c) masturbates; or (d) any other act of lewdness.