Can you stream music from an iPhone to an iPad?
If your iPad is a 3G/Wi-Fi model, then you could conceivably do this through the music app. The reason for the need for 3G is that in order to connect these devices there must be some sort of network present.  

That being said, this seems like overkill. If you are using your iPad for music and only music in the car, and if you have the means and desire to pull this off, then I would strongly suggest you simply get a car stereo with a 1/8" input auxiliary jack.  This would allow you to plug the iPhone directly into your car stereo, without any other hassle.   

Furthermore, streaming data like this is a good way to unnecessarily rack up data usage charges, as you would be transmitting data 100% of the time. Go with the plug-n-play solution: you stream no data at all.  

Seriously think about this. Again, it seems a bit like overkill.