I tried the 0.4 version and found a little issue:

1. Start mediaplayer and play some music
2. Turn on fm-carkit which start the fm tuner as well. The music is now played on the radio
3. On N900 make a phone call. The call works fine and the audio is indeed through the radio. So far so good.
3. End the call. The music immediately starts playing through the radio but lasts for only 2 seconds. After that the N900 starts playing the music through its internal speaker at a low volume. The fm transmitter is still 'on' but is sending only silence.
4. After about 15 seconds, the Fm transmitter suddenly stops working -> Mediaplayer's audio through the internal speakers suddenly goes to a high volume > Fm transmission restarts and radio starts the playing the audio again.

Keep the good work going!