I had a tiny mp3 player for my car, with an audio out and a usb charging port. It still couldn't charge and play at the same time. I was pretty upset after buying a cigarette lighter - USB adapter for it.
The first time I tried playing a device into the car radio, I had a bad experience - it was noisy to the point of being unlistenable. Later I discovered it was because I was charging it at the same time, unplug the charger and all was well.
