I wouldn't use an iPad in the car to play music, unless you can dock it, the chance of bumping the screen is high, slightly off topic I installed a fairly cheap Sony sound deck system that has an AUX port for iPod,iPhone or iPad playback, but more importantly has a USB port for flash memory sticks, I use the USB port for short trips and long trips you can use an IPad for music selection.
I previously had an fm tuner to convert music as stated in previous post but found it locked onto radio stations too easily.