Could I connect the D+ and D- to my car stereo's USB input, then connect the red and brown (+/-) to a 2amp cigarette plu'sg USB? My issue is that the USB in the head unit is low power and doesn't charge my iPad I use to play through it so I'm trying to figure out getting a high power source in there.
Would this work?
Depends on the radio and the charger.
If both have common ground, then you could connect D+, D- to the radio and power/ground to the charger. However, some cigarette lighter port power converters do not have common ground with the car, in that case you could blow up the radio, ipad or both.
You could also use a powered USB hub, assuming the radio supports them.
However, it may be that the iPad communicates with the radio, finds out that it cannot provide a lot of power and does not try to use more current than the radio allows.
The device must negotiate with the hub to receive higher currents. So no, it won't work, even if the radio and power supply have a common ground. The radio unit will not allow the device to take more current than it negotiates, and unless you hack the radio, it won't negotiate high current. You should buy a car charger and connect the headphone audio output to the radio unit to get high current charging and audio.
