ShR3K, if all you are looking for is a FM transmitter than the N900 has it already built in. All you need to do is set a frequency not used by any radio station in your car and set the same frequency on the N900 and your good to go.
The phone calls (once answered) can be heard through the speakers of your car. There is even a widget out there which allows you to turn the transmitter on with just a tap.

I don't know if this is what you meant but hope it helps.

p.s. The only problem with this system is that you have to keep the mobile close so that your voice can be heard.