I tested this out on two cars:

Acura RSX - 04
Toyota Camry SE - 08

In both car, the FM transmitter is definitely useless for the front seat. Once I put N900 behind the seat, then I could get clear sound.

BTW, no point of Fm Transmitter if you have to turn off bluetooth signal. My Camry has handsfree. I couldn't get FM signal as long as my N900's bluetooth is connected to the car.

I have an FM transmitter wire for the Acura. Therefore, I could just plug it into N900's 3.5mm jack for sound. It work fine. I don't have put the N900 behind my seat.

For the Camry, it has an aux port which I don't have to turn off BT.