It may not be the N900 itself. It might be the car recharge and the usb data cable which is the culprit. I have a nice little DC/AC power adapter which provices Fireware and USB powers. I use this to recharge my old phone and NITs. But when I tried using it with N900, it doesn't work. The N900 doesn't kick into recharge mode. And like the OP, it's kills my FM Transmitter. After I disconnect it, it seems the FM transmitter power output is also drastically reduced. I have to reboot the N900 to FM transmitter back to normal.

I suspect that the data connections in the USB data cable are confusing USB hardware or drivers. Does anyone know if the AC recharge only has the power connections wired on USB port? If not how are the data connections wired?