Help! iPod touch FM Transmitter problem!

Hi! I bought a Griffin iTrip FM Transmitter for my car. I plugged it into the cigarette lighter, set my radio to a free station, and tried to play a song. But all that happens is the volume bar disappears on my iPod and no music will play through the actual car speakers. Anyone have a solution to this? I don't think it is the FM transmitter because I tried out a Monster one and the same exact thing happened...