Translators: they are a pox on the FM radio dial.
Translators, by definition, are small low-power FM radio stations licensed with power levels of up to 250 watts. Translators are licensed by the FCC as "relay stations" only: they may not originate their own programming and must rebroadcast the signal from a full-power "parent station."
Translators were originally designed to be used by FM radio stations located in difficult terrain, like mountainous regions of the United States, to help fill in gaps in their signal area.
Instead, several groups (most notably religious "pay to pray"-type broadcasters) have used the translator rules to build large networks of low-power radio stations across the country. Translators are easier to site and cheaper to build and operate than full-power FM radio stations.
Translators provide a unique target for the pirate broadcaster: because they're unmanned, they're not difficult to hijack. And because many (but not all) relay another FM radio signal, it's relatively easy to take them over temporarily - all it takes is a stronger signal than the parent station provides.
"Borrowing" a translator to broadcast a pirate signal has already been successfully done: below is a short synopsis of the process from Liquid Radio's Head Cheese. Such activity, as always, remains highly illegal in the eyes of the FCC.
Hacking a licensed FM translator is quite easy. There's no need to mess with wires or have to break and enter into the translator facility. Hacking an FM translator is perfect for a quick broadcast and a quick getaway.
A translator is a relay station that rebroadcasts signals from another FM radio station. If a station is transmitting on 88.5 MHz, a translator will pick up the 88.5 MHz signal, and retransmit it over another FM frequency. Example, the translator could be receiving a signal on 88.5 MHz, and retransmitting it on 104.1 MHz.
Broadcasting an "alternative" signal over the translator is pretty simple. The first step is to obtain a low power FM transmitter. The transmitter can send out less then 100 milliwatts (.1 watt) and work effectively. I recommend a Ramsey model transmitter because they can be powered easily off of a car cigarette lighter. Since the power of the transmitter is so low, you can use a simple TV "rabbit ear" antenna to radiate the signal.
The second step is to find the location of the "guinea pig" translator. Set up your transmitter about 1/4 to 1/2 mile from the translator location. Tune your low power FM transmitter to transmit on the frequency of the station the translator is receiving. Turn on your transmitter and you should hear your signal over the translator, blocking out the other station.
As far as providing audio, just plug a portable CD player into your transmitter, and do your own show from the car, using someone else's broadcast facility.
When your broadcast is finished, just shut down your gear, pack it up, and drive away. Don't worry about being tracked down after you've shut down your signal. Since you're no longer broadcasting, you can't be tracked, and the translator will have begun rebroadcasting its original station feed.
I recommend that you keep your broadcast short, like 30 minutes to 1 hour. That way you can do the deed and still get away. It generally takes longer than that for the authorities to find out where you are operating from. If you decide to repeat this activity, try a different location around the vicinity of the translator each time. This greatly reduces the risk of getting caught.