Great except FM transmitter

I have an older Sirius radio that works great with FM transmission. I bought the Sportster for a new car. The features are great, but the FM transmission is much too weak to be worthwhile. I thought there was something wrong with the radio, and exchanged it for another, but the FM transmission was still very poor with the second radio, and essentially non-functional. This unit must be hard wired into the car radio to work.