Thanks, this helped me!

I made some progress:

• Store the original sound settings before routing the sound,
• "Phone number can be transmitted by RDS to the radio BEFORE accepting the call." (not tested, will I do tommorow morning while driving to work)
  
• Most important:
  When the application is running and a incoming or outgoing call is initiated:
  • fmtxd is started
  • routing through the fm-transmitter will be done automatically (no need for enabling the speaker and hitting the button)

So in fact the fm-carkit is doing what it supposed to be doing, act as a carkit

Next step is a nice widget or adding a profile to the phone settings.

Again, source is attached for people interrested. I will look into adding this to the repositories, so it can be installed with application manager.