Use iPod touch to connect phone to car stereo
Bluetooth devices are designed to function under certain "profiles". This is what allows a headset to work with Skype versus Bluetooth headphones which might work with iTunes. Apple's iOS uses very restrictive setup with their Bluetooth profiles. Because of this you cannot pair an iPod touch to a mobile phone.

Here is a list of the supported Bluetooth profiles: http://support.apple.com/kb/ht3647