Did you ever find a solution to this? I'm looking for the same thing. My car has a USB port to plug in my iPhone for pandora streaming and to play other music. The problem is that port only offers about 0.5A of charge current and seems to be screwing up my iPhone 6s's battery life by continuously providing it with a trickle of charge.
I want to split the lightning port cable so that one usb plugs into my 12v usb charger (when a charge is needed) and the other sends the music signal to the car's usb plug for music connection.
(Btw, I want to use the car's usb plug for music since it's the only way to integrate pandora in the car with iPhone [android uses Bluetooth, but not iphone]. Also, the sound quality is far superior using usb]