To be fair, you could control anything with the N900.

• Do what's been suggested and use a micro as a bluetooth-to-RF gateway
• Do similar and use a micro as an IR-to-RF gateway with the N900 IR transmitter.
• Do similar once more and use a micro as an FM-to-another-frequency gateway with fmtx
• Do another similar thing and use a micro as a USB-to-RF gateway, with the N900 in host mode
• Make an IP-to-RF gateway and do it over the network interface

The reason the N900 is more flexible than a lot of other devices is because of the number of I/O interfaces it supports; in addition to the more common GSM/WLAN/BT, there's also IR xmit, FM receive/xmit, USB client/host mode, audio output, composite video output.. be creative! Of course it would be pretty cool for the FM transmitter to be a software-defined radio and be able to flash it to operate at 40MHz, but it's probably the most flexible phone on the market as-is.

Also worth noting that any of the above would resolve the range issues with bluetooth/fmtx/IR, because the micro would just have to be in range of the N900 and the car - and at 40MHz it's distance from the car would be greater than bluetooth - rather than the phone itself needing to be within range of the car.