Does your car have an auxiliary jack? Newer cars have them; some have USB as well. You can use either to plug in your iPad so that the sound plays out of your car's speakers, same as with any MP3 player. If your car lacks either, you can have the stereo replaced to add one with an auxiliary jack and/or USB plug. You can buy an auxiliary jack cable for a few bucks almost anywhere, like at Radio Shack, Best Buy, Target, etc.
My cars have both auxiliary jacks and USB, but I don't notice any difference in sound quality. YMMV.
If you can't or don't want to plug into your car stereo, you might consider a Bluetooth speaker. I recommend a Jambox. 200 bucks. Great sound, easily portable and user friendly. Good battery life.
You also could consider an FM transmitter if your car lacks auxiliary and USB outlets. I've tried only one, though, received as a gift years ago. It was supposed to be a good one, but I still found sound quality spotty (depends on interference, radio frequency use where you are). For the price of a good transmitter weighed against more flexible uses, I'd still go with something like a Jambox instead. Again, YMMV.
Something else you might consider:
http://www.cyberguys.com/product-details/?productid=56018
No batteries required, and cheaper than a Jambox.
My experience with the iPhone version of the horn stand: It does amplify sound, but it sounds crummy, like you're using an aluminum can to amplify. It does work for phone calls while I drive, though, when loudness is more important than sound quality (unlike with music). Plus, I paid like 4 bucks for the iPhone version from Amazon.
I don't know whether I ended up with a knockoff, because the iPhone ones typically retail for 30 bucks, I think. Maybe I have a knockoff, and maybe the original sounds better. YMMV.
