Frequency Modulation is a unique animal. An FM receiver will hear only the strongest signal, blanking out any other signals totally. This is why aircraft do not use FM for voice communications, even though they are smack in the FM frequency bands; better to hear some station interference with AM than to lose touch with an aircraft totally.
But FM is the best choice for analog transmission of high-quality music. You may have an excellent FM transmitter, but if it is overpowered by a local broadcaster it's going to sound crappy. The suggestion of playing with your car's antenna is a good one, but nothing will beat a direct hardwired connection.