Perhaps you could decrease the sensitivity of your radio reciever to reduce the background noise? If you have a local/distance (DX/LO sometimes) selector on the radio, set it to local; as this will reduce the sensitivity of the reciever. You can also try to position the transmitter very close to the car's antenna, or perhaps put it extremely close to the actual head unit. IF you do not regularly use the radio, you might consider disconnecting the antenna.
BTW, does anyone know of any headunits that have a real AUX in (that doesn't need an expensive adapter or other circuitry to trick it into working). To me, that would be the best feature ever (and a 1/8" line on on the front would be even bettter).