Many cars are equipped with a service called Radio Data System, the readout that tells you the name of the radio station, the artist and the song you’re listening to. That textual information is added to the FM signal with a special encoder. Stations can choose to broadcast whatever text they choose so long as it fits within the limited number of display characters. Think of it as Twitter for FM.
And like many other data systems, RDS can be hacked.
Two Michigan public radio stations, WVGR-FM and WFUM-FM had their RDS hacked, resulting in the stations sending out obscene messages to dashboards and other RDS-enabled radios instead of artist and song information. Both stations are licensed to the University of Michigan, which, unfortunately, were holding pledge drives at the time. Oops.
Read more about what happened. (Radio engineers and PDs take note!)