I'm not sure what you are trying to accomplish is doable, and it's unlikely that it will be. The Win7 phones, at this time, don't have a standard Zune connector, which has specific pinouts for different functions, such as, passing audio, charging, basically controlling functions, etc. (much like the pinout on the iPods/iPhone)
Since all of the phones, at this time, have just USB mini/micro connectors, there has to be a driver associated on the receiver/computer/other device that knows how to communicate with the phone, process what you're trying to accomplish, and basically have digital two-way communication.
The more likely way, is to have a bluetooth enabled head deck, that allows two-way control (I'm pretty sure these are out, not sure about the two way), and a separate car charger...
Anyone else have any input that might help?