Convert low power USB to high power USB in car stereo.
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.